Python Programming FAQ
amk@nyman.amk.ca
amk@nyman.amk.ca
Sat, 16 Aug 2003 15:46:33 -0400
Here's another one of the new FAQ documents, about basic Python
programming and data types.
--amk
====================================
Programming FAQ
====================================
:Date: $Date: 2003/08/14 20:49:08 $
:Version: $Revision: 1.8 $
:Web site: http://www.python.org/
.. contents::
.. sectnum::
General Questions
===========================
Is there a source code level debugger with breakpoints, single-stepping, etc.?
-------------------------------------------------------------------------------
Yes.
The pdb module is a simple but adequate console-mode debugger for
Python. It is part of the standard Python library, and is `documented
in the Library Reference Manual
<http://www.python.org/doc/current/lib/module-pdb.html>`_. You can
also write your own debugger by using the code for pdb as an example.
The IDLE interactive development environment, which is part of the
standard Python distribution (normally available as Tools/scripts/idle),
includes a graphical debugger. There is documentation for the IDLE
debugger at http://www.python.org/idle/doc/idle2.html#Debugger
PythonWin is a Python IDE that includes a GUI debugger based on pdb.
The Pythonwin debugger colors breakpoints and has quite a few cool
features such as debugging non-Pythonwin programs. A reference
can be found at
http://www.python.org/ftp/python/pythonwin/pwindex.html More
recent versions of PythonWin are available as a part of the
ActivePython distribution (see
http://www.activestate.com/Products/ActivePython/index.html).
Pydb is a version of the standard Python debugger pdb, modified for
use with DDD (Data Display Debugger), a popular graphical debugger
front end. Pydb can be found at
http://packages.debian.org/unstable/devel/pydb.html> and DDD can be
found at http://www.gnu.org/software/ddd.
There are a number of commmercial Python IDEs that include graphical
debuggers. They include:
* Wing IDE (http://wingide.com)
* Komodo IDE (http://www.activestate.com/Products/Komodo)
Is there a tool to help find bugs or perform static analysis?
----------------------------------------------------------------------
Yes. PyChecker is a static analysis tool that finds bugs in Python
source code and warns about code complexity and style. You
can get PyChecker from http://pychecker.sf.net.
How can I create a stand-alone binary from a Python script?
-------------------------------------------------------------------
You don't need the ability to compile Python to C code, if all you
want is a stand-alone program that users can download and run without
having to install the Python distribution first. There are a number
of tools that determine the set of modules required by a program and
bind these modules together with a Python binary to produce a single
executable.
One is to use the freeze tool, which is included in the Python
source tree as ``Tools/freeze``. It converts Python byte
code to C arrays; a C compiler you can embed all
your modules into a new program, which is then linked
with the standard Python modules.
It works by scanning your source recursively for import statements (in
both forms) and looking for the modules in the standard Python path as
well as in the source directory (for built-in modules). It then turns
the bytecode for modules written in Python into C code (array
initializers that can be turned into code objects using the marshal
module) and creates a custom-made config file that only contains those
built-in modules which are actually used in the program. It then
compiles the generated C code and links it with the rest of the Python
interpreter to form a self-contained binary which acts exactly like
your script.
Obviously, freeze requires a C compiler. There are several other
utilities which don't. The first is Gordon McMillan's installer at
http://www.mcmillan-inc.com/install1.html
which works on Windows, Linux and at least some forms of Unix.
Another is Thomas Heller's py2exe (Windows only) at
http://starship.python.net/crew/theller/py2exe
A third is Christian Tismer's `SQFREEZE
<http://starship.python.net/crew/pirx>`_ which appends the byte code
to a specially-prepared Python interpreter that can find the byte
code in the executable.
A fourth is Fredrik Lundh's `Squeeze <http://www.pythonware.com/products/python/squeeze>`_.
Are there coding standards or a style guide for Python programs?
------------------------------------------------------------------------
Yes. The coding style required for standard library modules
is documented as `PEP 8 <http://www.python.org/peps/pep-0008.html>`_.
My program is too slow. How do I speed it up?
----------------------------------------------------
That's a tough one, in general. There are many tricks to speed up
Python code; consider rewriting parts in C as a last
resort.
One thing to notice is that function and (especially) method calls are
rather expensive; if you have designed a purely OO interface with lots
of tiny functions that don't do much more than get or set an instance
variable or call another method, you might consider using a more
direct way such as directly accessing instance variables. Also see the
standard module "profile" (`described in the Library Reference manual
<http://www.python.org/doc/current/lib/module-profile.html>`_) which
makes it possible to find out where your program is spending most of
its time (if you have some patience -- the profiling itself can slow
your program down by an order of magnitude).
Remember that many standard optimization heuristics you
may know from other programming experience may well apply
to Python. For example it may be faster to send output to output
devices using larger writes rather than smaller ones in order to
reduce the overhead of kernel system calls. Thus CGI scripts
that write all output in "one shot" may be faster than
those that write lots of small pieces of output.
Also, be sure to use Python's core features where appropriate.
For example, slicing allows programs to chop up
lists and other sequence objects in a single tick of the interpreter's
mainloop using highly optimized C implementations. Thus to
get the same effect as::
L2 = []
for i in range[3]:
L2.append(L1[i])
it is much shorter and far faster to use ::
L2 = list(L1[:3]) # "list" is redundant if L1 is a list.
Note that the functionally-oriented builtins such as
``map()``, ``zip()``, and friends can be a convenient
accelerator for loops that perform a single task. For example to pair the elements of two
lists together::
>>> zip([1,2,3], [4,5,6])
[(1, 4), (2, 5), (3, 6)]
or to compute a number of sines::
>>> map( math.sin, (1,2,3,4))
[0.841470984808, 0.909297426826, 0.14112000806, -0.756802495308]
The operation completes very quickly in such cases.
Other examples include the ``join()`` and ``split()``
methods of string objects. For example if s1..s7 are large (10K+) strings then
``"".join([s1,s2,s3,s4,s5,s6,s7])` may be far faster than
the more obvious ``s1+s2+s3+s4+s5+s6+s7``, since the "summation"
will compute many subexpressions, whereas ``join()`` does all the
copying in one pass. For manipulating strings, use
the ``replace()`` method on string objects. Use
regular expressions only when you're not dealing with constant string patterns.
Consider using the string formatting operations
``string % tuple`` and ``string % dictionary``.
Be sure to use the ``list.sort()`` builtin method to do sorting, and see
the `sorting mini-HOWTO <http://www.amk.ca/python/howto/sorting/XXX>`_ for examples of moderately advanced usage.
``list.sort()`` beats other techniques for sorting in all but the most
extreme circumstances.
Another common trick is to "push loops into functions or methods."
For example suppose you have a program that runs slowly and you
use the profiler to determine that a Python function ``ff()``
is being called lots of times. If you notice that ``ff ()``::
def ff(x):
...do something with x computing result...
return result
tends to be called in loops like::
list = map(ff, oldlist)
or::
for x in sequence:
value = ff(x)
...do something with value...
then you can often eliminate function call overhead by rewriting
``ff()`` to::
def ffseq(seq):
resultseq = []
for x in seq:
...do something with x computing result...
resultseq.append(result)
return resultseq
and rewrite the two examples to ``list = ffseq(oldlist)`` and to::
for value in ffseq(sequence):
...do something with value...
Single calls to ff(x) translate to ffseq([x])[0] with little
penalty. Of course this technique is not always appropriate
and there are other variants which you can figure out.
You can gain some performance by explicitly storing the results of
a function or method lookup into a local variable. A loop like::
for key in token:
dict[key] = dict.get(key, 0) + 1
resolves dict.get every iteration. If the method isn't going to
change, a slightly faster implementation is::
dict_get = dict.get # look up the method once
for key in token:
dict[key] = dict_get(key, 0) + 1
Default arguments can be used to determine values once, at
compile time instead of at run time. This can only be done for
functions or objects which will not be changed during program
execution, such as replacing ::
def degree_sin(deg):
return math.sin(deg * math.pi / 180.0)
with ::
def degree_sin(deg, factor = math.pi/180.0, sin = math.sin):
return sin(deg * factor)
Because this trick uses default arguments for terms which should
not be changed, it should only be used when you are not concerned
with presenting a possibly confusing API to your users.
Don't bother applying these optimization tricks until you know you
need them, after profiling has indicated that a particular function is
the heavily executed hot spot in the code. Optimizations almost
always make the code less clear, and you shouldn't pay the costs
of reduced clarity (increased development time, greater likelihood of bugs)
unless the resulting performance benefit is worth it.
For an anecdote related to optimization, see
http://www.python.org/doc/essays/list2str.html.
Core Language
==================
How do you set a global variable in a function?
----------------------------------------------------------
Did you do something like this? ::
x = 1 # make a global
def f():
print x # try to print the global
...
for j in range(100):
if q>3:
x=4
Any variable assigned in a function is local to that function.
unless it is specifically declared global. Since a value is bound
to ``x`` as the last statement of the function body, the compiler
assumes that ``x`` is local. Consequently the ``print x``
attempts to print an uninitialized local variable and will
trigger a ``NameError``.
The solution is to insert an explicit global declaration at the start
of the function::
def f():
global x
print x # try to print the global
...
for j in range(100):
if q>3:
x=4
In this case, all references to ``x`` are interpreted as references
to the ``x`` from the module namespace.
What are the rules for local and global variables in Python?
--------------------------------------------------------------------------
In Python, variables that are only referenced inside a function are
implicitly global. If a variable is assigned a new value anywhere
within the function's body, it's assumed to be a local. If a variable
is ever assigned a new value inside the function, the variable is
implicitly local, and you need to explicitly declare it as 'global'.
Though a bit surprising at first, a moment's consideration explains
this. On one hand, requiring ``global`` for assigned variables provides
a bar against unintended side-effects. On the other hand, if ``global``
was required for all global references, you'd be using ``global`` all the
time. You'd have to declare as global every reference to a
builtin function or to a component of an imported module. This
clutter would defeat the usefulness of the ``global`` declaration for
identifying side-effects.
How do I share global variables across modules?
------------------------------------------------
The canonical way to share information across modules within a single
program is to create a special module (often called config or cfg).
Just import the config module in all modules of your application; the
module then becomes available as a global name. Because there is only
one instance of each module, any changes made to the module object get
reflected everywhere. For example:
config.py::
x = 0 # Default value of the 'x' configuration setting
mod.py::
import config
config.x = 1
main.py::
import config
import mod
print config.x
Note that using a module is also the basis for implementing the
Singleton design pattern, for the same reason.
What are the "best practices" for using import in a module?
------------------------------------------------------------------------------
In general, don't use ``from modulename import *``.
Doing so clutters the importer's namespace. Some people avoid this idiom
even with the few modules that were designed to be imported in this
manner. Modules designed in this manner include ``Tkinter``,
``threading``, and ``wxPython``.
Import modules at the top of a file. Doing so makes it clear what
other modules your code requires and avoids questions of whether the
module name is in scope. Using one import per line makes it easy to
add and delete module imports, but using multiple imports per line
uses less screen space.
It's good practice if you import modules in the following order:
1. standard libary modules -- e.g. ``sys``, ``os``, ``getopt``, ``re``)
2. third-party library modules (anything installed in Python's
site-packages directory) -- e.g. mx.DateTime, ZODB, PIL.Image, etc.
3. locally-developed modules
Never use relative package imports. If you're writing code that's
in the ``package.sub.m1`` module and want to import ``package.sub.m2``,
do not just write ``import m2``, even though it's legal.
Write ``from package.sub import m2`` instead. Relative imports can lead to a
module being initialized twice, leading to confusing bugs.
It is sometimes necessary to move imports to a function or class to
avoid problems with circular imports. Gordon McMillan says:
Circular imports are fine where both modules use the "import <module>"
form of import. They fail when the 2nd module wants to grab a name
out of the first ("from module import name") and the import is at
the top level. That's because names in the 1st are not yet available,
because the first module is busy importing the 2nd.
In this case, if the second module is only used in one function, then the
import can easily be moved into that function. By the time the import
is called, the first module will have finished initializing, and the
second module can do its import.
It may also be necessary to move imports out of the top level of code
if some of the modules are platform-specific. In that case, it may
not even be possible to import all of the modules at the top of the
file. In this case, importing the correct modules in the
corresponding platform-specific code is a good option.
Only move imports into a local scope, such as inside a function
definition, if it's necessary to solve a problem such as avoiding a
circular import or are trying to reduce the initialization time of a
module. This technique is especially helpful if many of the imports
are unnecessary depending on how the program executes. You may also
want to move imports into a function if the modules are only ever used
in that function. Note that loading a module the first time may be
expensive because of the one time initialization of the module, but
loading a module multiple times is virtually free, costing only a couple of
dictionary lookups. Even if the module name has gone out of scope,
the module is probably available in sys.modules.
If only instances of a specific class use a module, then it is
reasonable to import the module in the class's ``__init__`` method and
then assign the module to an instance variable so that the module is
always available (via that instance variable) during the life of the
object. Note that to delay an import until the class is instantiated,
the import must be inside a method. Putting the import inside the
class but outside of any method still causes the import to occur when
the module is initialized.
How can I pass optional or keyword parameters from one function to another?
-------------------------------------------------------------------------------
Collect the arguments using the ``*`` and ``**`` specifiers in the function's
parameter list; this gives you the positional arguments as a tuple
and the keyword arguments as a dictionary. You can
then pass these arguments when calling another function by using
``*`` and ``**``::
def f(x, *tup, **kwargs):
...
kwargs['width']='14.3c'
...
g(x, *tup, **kwargs)
In the unlikely case that you care about Python
versions older than 2.0, use 'apply'::
def f(x, *tup, **kwargs):
...
kwargs['width']='14.3c'
...
apply(g, (x,)+tup, kwargs)
How do I write a function with output parameters (call by reference)?
-----------------------------------------------------------------------------
Remember that arguments are passed by assignment in Python. Since
assignment just creates references to objects, there's no alias
between an argument name in the caller and callee, and so no
call-by-reference per se. You can achieve the desired effect in a
number of ways.
1) By returning a tuple of the results::
def func2(a, b):
a = 'new-value' # a and b are local names
b = b + 1 # assigned to new objects
return a, b # return new values
x, y = 'old-value', 99
x, y = func2(x, y)
print x, y # output: new-value 100
This is almost always the clearest solution.
2) By using global variables. This isn't thread-safe, and is not
recommended.
3) By passing a mutable (changeable in-place) object::
def func1(a):
a[0] = 'new-value' # 'a' references a mutable list
a[1] = a[1] + 1 # changes a shared object
args = ['old-value', 99]
func1(args)
print args[0], args[1] # output: new-value 100
4) By passing in a dictionary that gets mutated::
def func3(args):
args['a'] = 'new-value' # args is a mutable dictionary
args['b'] = args['b'] + 1 # change it in-place
args = {'a':' old-value', 'b': 99}
func3(args)
print args['a'], args['b']
5) Or bundle up values in a class instance::
class callByRef:
def __init__(self, **args):
for (key, value) in args.items():
setattr(self, key, value)
def func4(args):
args.a = 'new-value' # args is a mutable callByRef
args.b = args.b + 1 # change object in-place
args = callByRef(a='old-value', b=99)
func4(args)
print args.a, args.b
There's almost never a good reason to get this complicated.
Your best choice is to return a tuple containing the multiple results.
How do you make a higher order function in Python?
----------------------------------------------------------
You have two choices: you can use nested scopes
or you can use callable objects. For example, suppose you wanted to
define ``linear(a,b)`` which returns a function ``f(x)`` that computes the
value ``a*x+b``. Using nested scopes::
def linear(a,b):
def result(x):
return a*x + b
return result
Or using a callable object::
class linear:
def __init__(self, a, b):
self.a, self.b = a,b
def __call__(self, x):
return self.a * x + self.b
In both cases::
taxes = linear(0.3,2)
gives a callable object where taxes(10e6) == 0.3 * 10e6 + 2.
The callable object approach has the disadvantage that it is a bit
slower and results in slightly longer code. However, note that a
collection of callables can share their signature via inheritance::
class exponential(linear):
# __init__ inherited
def __call__(self, x):
return self.a * (x ** self.b)
Object can encapsulate state for several methods::
class counter:
value = 0
def set(self, x): self.value = x
def up(self): self.value=self.value+1
def down(self): self.value=self.value-1
count = counter()
inc, dec, reset = count.up, count.down, count.set
Here ``inc()``, ``dec()`` and ``reset()`` act like functions which share the
same counting variable.
How do I copy an object in Python?
------------------------------------------
In general, try copy.copy() or copy.deepcopy() for the general case. Not all
objects can be copied, but most can.
Some objects can be copied more easily.
Dictionaries have a ``copy()`` method::
newdict = olddict.copy()
Sequences can be copied by slicing::
new_l = l[:]
How can I find the methods or attributes of an object?
--------------------------------------------------------------
For an instance x of a user-defined class, ``dir(x)`` returns an
alphabetized list of the names containing the instance attributes and
methods and attributes defined by its class.
How can my code discover the name of an object?
-------------------------------------------------------
Generally speaking, it can't, because objects don't really have
names. Essentially, assignment always binds a name to a value; The
same is true of ``def`` and ``class`` statements, but in that case the
value is a callable. Consider the following code::
class A:
pass
B = A
a = B()
b = a
print b
<__main__.A instance at 016D07CC>
print a
<__main__.A instance at 016D07CC>
Arguably the class has a name: even though it is bound to two names
and invoked through the name B the created instance is still reported
as an instance of class A. However, it is impossible to say whether
the instance's name is a or b, since both names are bound to the same
value.
Generally speaking it should not be necessary for your code to "know
the names" of particular values. Unless you are deliberately writing
introspective programs, this is usually an indication that a change of
approach might be beneficial.
In comp.lang.python, Fredrik Lundh once gave an excellent analogy in
answer to this question:
The same way as you get the name of that cat you found on your
porch: the cat (object) itself cannot tell you its name, and it
doesn't really care -- so the only way to find out what it's called
is to ask all your neighbours (namespaces) if it's their cat
(object)...
....and don't be surprised if you'll find that it's known by many
names, or no name at all!
Is there an equivalent of C's "?:" ternary operator?
----------------------------------------------------------------------
No. In many cases you can mimic a?b:c with "a and b or
c", but there's a flaw: if b is zero (or empty, or None -- anything
that tests false) then c will be selected instead. In many cases you
can prove by looking at the code that this can't happen (e.g. because
b is a constant or has a type that can never be false), but in general
this can be a problem.
Tim Peters (who wishes it was Steve Majewski) suggested the following
solution: (a and [b] or [c])[0]. Because [b] is a singleton list it
is never false, so the wrong path is never taken; then applying [0] to
the whole thing gets the b or c that you really wanted. Ugly, but it
gets you there in the rare cases where it is really inconvenient to
rewrite your code using 'if'.
The best course is usually to write a simple ``if...else`` statement.
Another solution is to implement the "?:" operator as a function::
def q(cond,on_true,on_false):
if cond:
if not isfunction(on_true): return on_true
else: return apply(on_true)
else:
if not isfunction(on_false): return on_false
else: return apply(on_false)
In most cases you'll pass b and c directly: ``q(a,b,c)``. To avoid
evaluating b or c when they shouldn't be, encapsulate them within a
lambda function, e.g.: ``q(a,lambda: b, lambda: c)``.
It has been asked *why* Python has no if-then-else expression.
There are several answers: many languages do
just fine without one; it can easily lead to less readable code;
no sufficiently "Pythonic" syntax has been discovered; a search
of the standard library found remarkably few places where using an
if-then-else expression would make the code more understandable.
In 2002, `PEP 308 <http://www.python.org/peps/pep-0308.html>`_ was
written proposing several possible syntaxes and the community was
asked to vote on the issue. The vote was inconclusive. Most people
liked one of the syntaxes, but also hated other syntaxes; many votes
implied that people preferred no ternary operator
rather than having a syntax they hated.
Is it possible to write obfuscated one-liners in Python?
----------------------------------------------------------------
Yes. Usually this is done by nesting `lambda` within `lambda`.
See the following three examples, due to Ulf Bartelt::
# Primes < 1000
print filter(None,map(lambda y:y*reduce(lambda x,y:x*y!=0,
map(lambda x,y=y:y%x,range(2,int(pow(y,0.5)+1))),1),range(2,1000)))
# First 10 Fibonacci numbers
print map(lambda x,f=lambda x,f:(x<=1) or (f(x-1,f)+f(x-2,f)): f(x,f),
range(10))
# Mandelbrot set
print (lambda Ru,Ro,Iu,Io,IM,Sx,Sy:reduce(lambda x,y:x+y,map(lambda y,
Iu=Iu,Io=Io,Ru=Ru,Ro=Ro,Sy=Sy,L=lambda yc,Iu=Iu,Io=Io,Ru=Ru,Ro=Ro,i=IM,
Sx=Sx,Sy=Sy:reduce(lambda x,y:x+y,map(lambda x,xc=Ru,yc=yc,Ru=Ru,Ro=Ro,
i=i,Sx=Sx,F=lambda xc,yc,x,y,k,f=lambda xc,yc,x,y,k,f:(k<=0)or (x*x+y*y
>=4.0) or 1+f(xc,yc,x*x-y*y+xc,2.0*x*y+yc,k-1,f):f(xc,yc,x,y,k,f):chr(
64+F(Ru+x*(Ro-Ru)/Sx,yc,0,0,i)),range(Sx))):L(Iu+y*(Io-Iu)/Sy),range(Sy
))))(-2.1, 0.7, -1.2, 1.2, 30, 80, 24)
# \___ ___ \___ ___ | | |__ lines on screen
# V V | |______ columns on screen
# | | |__________ maximum of "iterations"
# | |_________________ range on y axis
# |____________________________ range on x axis
Don't try this at home, kids!
Numbers and strings
==========================
How do I specify hexadecimal and octal integers?
--------------------------------------------------------
To specify an octal digit, precede the octal value with a zero. For
example, to set the variable "a" to the octal value "10" (8 in
decimal), type::
>>> a = 010
>>> a
8
Hexadecimal is just as easy. Simply precede the hexadecimal number with a
zero, and then a lower or uppercase "x". Hexadecimal digits can be specified
in lower or uppercase. For example, in the Python interpreter::
>>> a = 0xa5
>>> a
165
>>> b = 0XB2
>>> b
178
How do I convert a string to a number?
----------------------------------------------
For integers, use the built-in ``int()`` type constructor,
e.g. int('144') == 144. Similarly, ``float()`` converts to
floating-point, e.g. ``float('144') == 144.0``.
By default, these interpret the number as decimal, so that
``int('0144') == 144`` and ``int('0x144')`` raises
``ValueError``. ``int(string, base)`` takes the base to convert from
as a second optional argument, so ``int('0x144', 16) == 324``. If the
base is specified as 0, the number is interpreted using Python's
rules: a leading '0' indicates octal, and '0x' indicates a hex number.
Do not use the built-in function ``eval()`` if all you need is to
convert strings to numbers. ``eval()`` will be significantly slower
and it presents a security risk: someone could pass you a Python
expression that might have unwanted side effects. For example,
someone could pass ``__import__('os').system("rm -rf $HOME")`` which
would erase your home directory.
``eval()`` also has the effect of interpreting numbers as Python
expressions, so that e.g. eval('09') gives a syntax error because Python
regards numbers starting with '0' as octal (base 8).
How do I convert a number to a string?
----------------------------------------------
To convert, e.g., the number 144 to the string '144', use the built-in
function ``str()``. If you want a hexadecimal or octal
representation, use the built-in functions ``hex()`` or ``oct()``.
For fancy formatting, use `the % operator <http://www.python.org/doc/lib/typesseq-strings.html>`_ on strings, e.g. ``"%04d" % 144``
yields '0144' and ``"%.3f" % (1/3.0)`` yields '0.333'. See the library
reference manual for details.
How do I modify a string in place?
------------------------------------------
You can't, because strings are immutable. If you need an object with
this ability, try converting the string to a list or use the array
module::
>>> s = "Hello, world"
>>> a = list(s)
>>> print a
['H', 'e', 'l', 'l', 'o', ',', ' ', 'w', 'o', 'r', 'l', 'd']
>>> a[7:] = list("there!")
>>> ''.join(a)
'Hello, there!'
>>> import array
>>> a = array.array('c', s)
>>> print a
array('c', 'Hello, world')
>>> a[0] = 'y' ; print a
array('c', 'yello world')
>>> a.tostring()
'yello, world'
How do I use strings to call functions/methods?
----------------------------------------------------------
There are various techniques.
* The best is to use a dictionary that maps strings to functions. The
primary advantage of this technique is that the strings do not need
to match the names of the functions. This is also the primary
technique used to emulate a case construct::
def a():
pass
def b():
pass
dispatch = {'go': a, 'stop': b} # Note lack of parens for funcs
dispatch[get_input()]() # Note trailing parens to call function
* Use the built-in function ``getattr()``::
import foo
getattr(foo, 'bar')()
Note that getattr() works on any object, including classes, class
instances, modules, and so on.
This is used in several places in the standard library, like
this::
class Foo:
def do_foo(self):
...
def do_bar(self):
...
f = getattr(foo_instance, 'do_' + opname)
f()
* Use ``locals()`` or ``eval()`` to resolve the function name::
def myFunc():
print "hello"
fname = "myFunc"
f = locals()[fname]
f()
f = eval(fname)
f()
Note: Using ``eval()`` is slow and dangerous. If you don't have absolute control
over the contents of the string, someone could pass a string that
resulted in an arbitrary function being executed.
Is there an equivalent to Perl's chomp() for removing trailing newlines from strings?
--------------------------------------------------------------------------------------------
There are two partial substitutes. If you want to remove all trailing
whitespace, use the ``rstrip()`` method of string objects. This removes all trailing whitespace, not just a single newline.
Otherwise, if there is only one line in the string ``S``, use
``S.splitlines()[0]``.
Is there a scanf() or sscanf() equivalent?
--------------------------------------------------
Not as such.
For simple input parsing, the easiest approach is usually to split the
line into whitespace-delimited words using the ``split()`` method of
string objects and then convert decimal strings to numeric values using
``int()`` or ``float()``. ``split()`` supports an optional "sep"
parameter which is useful if the line uses something other than
whitespace as a separator.
For more complicated input parsing, regular expressions
more powerful than C's ``sscanf()`` and better suited for the task.
What does 'UnicodeError: ASCII [decoding,encoding] error: ordinal not in range(128)' mean?
-----------------------------------------------------------------------------------------------------
This error indicates that your Python installation can handle
only 7-bit ASCII strings. There are a couple ways to fix or
work around the problem.
If your programs must handle data in arbitary character set encodings,
the environment the application runs in will generally identify the
encoding of the data it is handing you. You need to convert the input
to Unicode data using that encoding. For example, a program that
handles email or web input will typically find character set encoding
information in Content-Type headers. This can then be used to
properly convert input data to Unicode. Assuming the string referred
to by ``value`` is encoded as UTF-8::
value = unicode(value, "utf-8")
will return a Unicode object. If the data is not correctly encoded as
UTF-8, the above call will raise a ``UnicodeError`` exception.
If you only want strings coverted to Unicode which have non-ASCII
data, you can try converting them first assuming an ASCII encoding,
and then generate Unicode objects if that fails::
try:
x = unicode(value, "ascii")
except UnicodeError:
value = unicode(value, "utf-8")
else:
# value was valid ASCII data
pass
It's possible to set a default encoding in a file called ``sitecustomize.py``
that's part of the Python library. However, this isn't recommended because changing the Python-wide default encoding may cause third-party extension modules to fail.
Note that on Windows, there is an encoding known as "mbcs", which uses
an encoding specific to your current locale. In many cases, and
particularly when working with COM, this may be an appropriate default
encoding to use.
Sequences (Tuples/Lists)
=================================
How do I convert between tuples and lists?
------------------------------------------------
The function ``tuple(seq)`` converts any sequence (actually, any
iterable) into a tuple with the same items in the same order.
For example, ``tuple([1, 2, 3])`` yields ``(1, 2, 3)`` and ``tuple('abc')``
yields ``('a', 'b', 'c')``. If the argument is
a tuple, it does not make a copy but returns the same object, so
it is cheap to call ``tuple()`` when you aren't sure that an object
is already a tuple.
The function ``list(seq)`` converts any sequence or iterable into a list with
the same items in the same order.
For example, ``list((1, 2, 3))`` yields ``[1, 2, 3]`` and ``list('abc')``
yields ``['a', 'b', 'c']``. If the argument is a list,
it makes a copy just like ``seq[:]`` would.
What's a negative index?
--------------------------------------------------------------------
Python sequences are indexed with positive numbers and
negative numbers. For positive numbers 0 is the first index
1 is the second index and so forth. For negative indices -1
is the last index and -2 is the pentultimate (next to last) index
and so forth. Think of ``seq[-n]`` as the same as ``seq[len(seq)-n]``.
Using negative indices can be very convenient. For example ``S[:-1]``
is all of the string except for its last character, which is useful
for removing the trailing newline from a string.
How do I iterate over a sequence in reverse order?
---------------------------------------------------------
If it is a list, the fastest solution is ::
list.reverse()
try:
for x in list:
"do something with x"
finally:
list.reverse()
This has the disadvantage that while you are in the loop, the list
is temporarily reversed. If you don't like this, you can make a copy.
This appears expensive but is actually faster than other solutions::
rev = list[:]
rev.reverse()
for x in rev:
<do something with x>
If it's not a list, a more general but slower solution is::
for i in range(len(sequence)-1, -1, -1):
x = sequence[i]
<do something with x>
A more elegant solution, is to define a class which acts as a sequence
and yields the elements in reverse order (solution due to Steve
Majewski)::
class Rev:
def __init__(self, seq):
self.forw = seq
def __len__(self):
return len(self.forw)
def __getitem__(self, i):
return self.forw[-(i + 1)]
You can now simply write::
for x in Rev(list):
<do something with x>
Unfortunately, this solution is slowest of all, due to the method
call overhead.
How do you remove duplicates from a list?
-------------------------------------------------
See the Python Cookbook for a long discussion of many ways to do this:
http://aspn.activestate.com/ASPN/Cookbook/Python/Recipe/52560
If you don't mind reordering the list, sort it and then scan from the
end of the list, deleting duplicates as you go::
if List:
List.sort()
last = List[-1]
for i in range(len(List)-2, -1, -1):
if last==List[i]: del List[i]
else: last=List[i]
If all elements of the list may be used as
dictionary keys (i.e. they are all hashable)
this is often faster ::
d = {}
for x in List: d[x]=x
List = d.values()
How do you make an array in Python?
----------------------------------------------------
Use a list::
["this", 1, "is", "an", "array"]
Lists are equivalent to C or Pascal arrays in their time complexity;
the primary difference is that a Python list can contain objects of
many different types.
The ``array`` module also provides methods for creating arrays of
fixed types with compact representations, but they are slower to index
than lists. Also note that the Numeric extensions and others define
array-like structures with various characteristics as well.
To get Lisp-style linked lists, you can emulate cons cells using tuples::
lisp_list = ("like", ("this", ("example", None) ) )
If mutability is desired, you could use lists instead of tuples. Here
the analogue of lisp car is ``lisp_list[0]`` and the analogue of cdr
is ``lisp_list[1]``. Only do this if you're sure you really need to,
because it's usually a lot slower than using Python lists.
How do I create a multidimensional list?
---------------------------------------------------------------
You probably tried to make a multidimensional array like this::
A = [[None] * 2] * 3
This looks correct if you print it::
>>> A
[[None, None], [None, None], [None, None]]
But when you assign a value, it shows up in multiple places:
>>> A[0][0] = 5
>>> A
[[5, None], [5, None], [5, None]]
The reason is that replicating a list with ``*`` doesn't create copies, it only creates references to the existing objects. The ``*3``
creates a list containing 3 references to the same list of length
two. Changes to one row will show in all rows, which is almost certainly
not what you want.
The suggested approach is to create a list of the desired length first
and then fill in each element with a newly created list::
A = [None]*3
for i in range(3):
A[i] = [None] * 2
This generates a list containing 3 different lists of length two.
You can also use a list comprehension::
w,h = 2,3
A = [ [None]*w for i in range(h) ]
Or, you can use an extension that provides a matrix datatype; `Numeric
Python <XXX>`_ is the best known.
How do I apply a method to a sequence of objects?
--------------------------------------------------------------------------
Use a list comprehension::
result = [obj.method() for obj in List]
More generically, you can try the following function::
def method_map(objects, method, arguments):
"""method_map([a,b], "meth", (1,2)) gives [a.meth(1,2), b.meth(1,2)]"""
nobjects = len(objects)
methods = map(getattr, objects, [method]*nobjects)
return map(apply, methods, [arguments]*nobjects)
Dictionaries
==================
How can I get a dictionary to display its keys in a consistent order?
-----------------------------------------------------------------------------
You can't. Dictionaries store their keys in an unpredictable order,
so the display order of a dictionary's elements will be similarly
unpredictable.
This can be frustrating if you want to save a printable version to a
file, make some changes and then compare it with some other printed
dictionary. In this case, use the ``pprint`` module to pretty-print
the dictionary; the items will be presented in order sorted by the key.
A more complicated solution is to subclass ``UserDict.UserDict``
to create a ``SortedDict`` class that prints itself in a predictable order.
Here's one simpleminded implementation of such a class::
import UserDict, string
class SortedDict(UserDict.UserDict):
def __repr__(self):
result = []
append = result.append
keys = self.data.keys()
keys.sort()
for k in keys:
append("%s: %s" % (`k`, `self.data[k]`))
return "{%s}" % string.join(result, ", ")
___str__ = __repr__
This will work for many common situations you might encounter, though
it's far from a perfect solution. The largest flaw is that if some
values in the dictionary are also dictionaries, their values won't be
presented in any particular order.
I want to do a complicated sort: can you do a Schwartzian Transform in Python?
--------------------------------------------------------------------------------------
Yes, it's quite simple with list comprehensions.
The technique, attributed to Randal Schwartz of the Perl community,
sorts the elements of a list by a metric which maps each element to
its "sort value". To sort a list of strings by their uppercase
values::
tmp1 = [ (x.upper(), x) for x in L ] # Schwartzian transform
tmp1.sort()
Usorted = [ x[1] for x in tmp1 ]
To sort by the integer value of a subfield extending from positions 10-15
in each string::
tmp2 = [ (int(s[10:15]), s) for s in L ] # Schwartzian transform
tmp2.sort()
Isorted = [ x[1] for x in tmp2 ]
Note that Isorted may also be computed by ::
def intfield(s):
return int(s[10:15])
def Icmp(s1, s2):
return cmp(intfield(s1), intfield(s2))
Isorted = L[:]
Isorted.sort(Icmp)
but since this method calls ``intfield()`` many times for each element
of L, it is slower than the Schwartzian Transform.
How can I sort one list by values from another list?
------------------------------------------------------------
Merge them into a single list of tuples, sort the resulting list,
and then pick out the element you want. ::
>>> list1 = ["what", "I'm", "sorting", "by"]
>>> list2 = ["something", "else", "to", "sort"]
>>> pairs = zip(list1, list2)
>>> pairs
[('what', 'something'), ("I'm", 'else'), ('sorting', 'to'), ('by', 'sort')]
>>> pairs.sort()
>>> result = [ x[1] for x in pairs ]
>>> result
['else', 'sort', 'to', 'something']
An alternative for the last step is::
result = []
for p in pairs: result.append(p[1])
If you find this more legible, you might prefer to use this instead of
the final list comprehension. However, it is almost twice as slow for
long lists. Why? First, the ``append()`` operation has to reallocate
memory, and while it uses some tricks to avoid doing that each time,
it still has to do it occasionally, and that costs quite a bit.
Second, the expression "result.append" requires an extra attribute
lookup, and third, there's a speed reduction from having to make
all those function calls.
Objects
=============
What is a class?
------------------------
A class is the particular object type created by executing
a class statement. Class objects are used as templates to create
instance objects, which embody both the data
(attributes) and code (methods) specific to a datatype.
A class can be based on one or more other classes, called its base
class(es). It then inherits the attributes and methods of its base
classes. This allows an object model to be successively refined by
inheritance. You might have a generic ``Mailbox`` class that provides
basic accessor methods for a mailbox, and subclasses such as
``MboxMailbox``, ``MaildirMailbox``, ``OutlookMailbox`` that handle
various specific mailbox formats.
What is a method?
-------------------------
A method is a function on some object ``x`` that you normally call as
``x.name(arguments...)``. Methods are defined as functions inside the
class definition::
class C:
def meth (self, arg):
return arg*2 + self.attribute
What is self?
---------------------
Self is merely a conventional name for the first argument of a method.
A method defined as ``meth(self, a, b, c)`` should be called as
``x.meth(a, b, c)`` for some instance ``x`` of the class in which the
definition occurs; the called method will think it is called as
``meth(x, a, b, c)``.
How do I check if an object is an instance of a given class or of a subclass of it?
-------------------------------------------------------------------------------------------
Use the built-in function ``isinstance(obj, cls)``. You can check if
an object is an instance of any of a number of classes by providing a tuple instead of a single class, e.g. ``isinstance(obj, (class1, class2, ...))``,
and can also check whether an object is one of Python's built-in types, e.g.
``isinstance(obj, str)`` or ``isinstance(obj, (int, long, float, complex))``.
Note that most programs do not use ``isinstance()`` on user-defined
classes very often. If you are developing the classes yourself, a
more proper object-oriented style is to define methods on the classes
that encapsulate a particular behaviour, instead of checking the
object's class and doing a different thing based on what class it is.
For example, if you have a function that does something::
def search (obj):
if isinstance(obj, Mailbox):
# ... code to search a mailbox
elif isinstance(obj, Document):
# ... code to search a document
elif ...
A better approach is to define a ``search()`` method on all the
classes and just call it::
class Mailbox:
def search(self):
# ... code to search a mailbox
class Document:
def search(self):
# ... code to search a document
obj.search()
What is delegation?
---------------------------
Delegation is an object oriented technique (also called a design
pattern). Let's say you have an object ``x`` and want to change the
behaviour of just one of its methods. You can create a new class that
provides a new implementation of the method you're interested in changing
and delegates all other methods to the corresponding method of ``x``.
Python programmers can easily implement delegation. For example, the
following class implements a class that behaves like a file but
converts all written data to uppercase::
class UpperOut:
def __init__(self, outfile):
self.__outfile = outfile
def write(self, s):
self.__outfile.write(s.upper())
def __getattr__(self, name):
return getattr(self.__outfile, name)
Here the ``UpperOut`` class redefines the ``write()`` method to
convert the argument string to uppercase before calling the underlying
``self.__outfile.write()`` method. All other methods are delegated to
the underlying ``self.__outfile`` object. The delegation is
accomplished via the ``__getattr__`` method; consult `the language
reference <http://www.python.org/doc/ref/attribute-access.html>`_ for
more information about controlling attribute access.
Note that for more general cases delegation can get trickier. When
attributes must be set as well as retrieved, the class must define a
``__settattr__`` method too, and it must do so carefully. The basic
implementation of __setattr__ is roughly equivalent to the following::
class X:
...
def __setattr__(self, name, value):
self.__dict__[name] = value
...
Most __setattr__ implementations must modify
self.__dict__ to store local state for self without
causing an infinite recursion.
How do I call a method defined in a base class from a derived class that overrides it?
----------------------------------------------------------------------------------------------
If you're using new-style classes, use the built-in ``super()`` function::
class Derived(Base):
def meth (self):
super(Derived, self).meth()
If you're using classic classes: For a class definition such as
``class Derived(Base): ...`` you can call method ``meth()`` defined in
``Base`` (or one of ``Base``'s base classes) as ``Base.meth(self,
arguments...)``. Here, ``Base.meth`` is an unbound method, so you
need to provide the ``self`` argument.
How can I organize my code to make it easier to change the base class?
------------------------------------------------------------------------------
You could define an alias for the base class, assign the real base
class to it before your class definition, and use the alias throughout
your class. Then all you have to change is the value assigned to the
alias. Incidentally, this trick is also handy if you want to decide
dynamically (e.g. depending on availability of resources) which base
class to use. Example::
BaseAlias = <real base class>
class Derived(BaseAlias):
def meth(self):
BaseAlias.meth(self)
...
How do I create static class data and static class methods?
-------------------------------------------------------------------
Static data (in the sense of C++ or Java) is easy; static methods
(again in the sense of C++ or Java) are not supported directly.
For static data, simply define a class attribute. To assign a new
value to the attribute, you have to explicitly use the class name in
the assignment::
class C:
count = 0 # number of times C.__init__ called
def __init__(self):
C.count = C.count + 1
def getcount(self):
return C.count # or return self.count
``c.count`` also refers to ``C.count`` for any ``c`` such that
``isinstance(c, C)`` holds, unless overridden by ``c`` itself or by some
class on the base-class search path from ``c.__class__`` back to ``C``.
Caution: within a method of C, an assignment like ``self.count = 42``
creates a new and unrelated instance vrbl named "count" in ``self``'s own dict.
Rebinding of a class-static data name must always specify the class
whether inside a method or not::
C.count = 314
Static methods are possible when you're using new-style classes::
class C:
def static(arg1, arg2, arg3):
# No 'self' parameter!
...
static = staticmethod(static)
However, a far more straightforward way to get the effect of a static
method is via a simple module-level function::
def getcount():
return C.count
If your code is structured so as to define one class (or tightly
related class hierarchy) per module, this supplies the desired
encapsulation.
How can I overload constructors (or methods) in Python?
---------------------------------------------------------------
This answer actually applies to all methods, but the question
usually comes up first in the context of constructors.
In C++ you'd write ::
class C {
C() { cout << "No arguments\n"; }
C(int i) { cout << "Argument is " << i << "\n"; }
}
in Python you have to write a single constructor that catches all
cases using default arguments. For example::
class C:
def __init__(self, i=None):
if i is None:
print "No arguments"
else:
print "Argument is", i
This is not entirely equivalent, but close enough in practice.
You could also try a variable-length argument list, e.g. ::
def __init__(self, *args):
....
The same approach works for all method definitions.
I try to use __spam and I get an error about _SomeClassName__spam.
--------------------------------------------------------------------------
Variables with double leading underscore are "mangled" to provide a
simple but effective way to define class private variables. Any
identifier of the form ``__spam`` (at least two leading
underscores, at most one trailing underscore) is textually
replaced with ``_classname__spam``, where ``classname`` is the
current class name with any leading underscores stripped.
This doesn't guarantee privacy: an outside user can still deliberately
access the "_classname__spam" attribute, and private values are visible
in the object's ``__dict__``. Many Python programmers never bother to use
private variable names at all.
My class defines __del__ but it is not called when I delete the object.
-------------------------------------------------------------------------------
There are several possible reasons for this.
The del statement does not necessarily call __del__ -- it simply
decrements the object's reference count, and if this reaches zero
__del__ is called.
If your data structures contain circular links (e.g. a tree where each
child has a parent reference and each parent has a list of children)
the reference counts will never go back to zero. Once in a while
Python runs an algorithm to detect such cycles, but the garbage
collector might run some time after the last reference to your data
structure vanishes, so your __del__ method may be called at an
inconvenient and random time. This is inconvenient if you're trying to
reproduce a problem. Worse, the order in which object's __del__
methods are executed is arbitrary. You can run ``gc.collect()`` to
force a collection, but there *are* pathological cases where objects will
never be collected.
Despite the cycle collector, it's still a good idea to define an
explicit ``close()`` method on objects to be called whenever you're
done with them. The ``close()`` method can then remove attributes
that refer to subobjecs. Don't call ``__del__`` directly --
``__del__`` should call ``close()`` and ``close()`` should make sure
that it can be called more than once for the same object.
Another way to avoid cyclical references is to use the "weakref"
module, which allows you to point to objects without incrementing
their reference count. Tree data structures, for instance, should use
weak references for their parent and sibling references (if they need
them!).
If the object has ever been a local variable in a function that caught
an expression in an except clause, chances are that a reference to the
object still exists in that function's stack frame as contained in the
stack trace. Normally, calling ``sys.exc_clear()`` will take care of
this by clearing the last recorded exception.
Finally, if your __del__ method raises an exception, a warning message
is printed to sys.stderr.
How do I get a list of all instances of a given class?
--------------------------------------------------------------
Python does not keep track of all instances of a class (or of a
built-in type). You can program the class's constructor to keep track
of all instances by keeping a list of weak references to each
instance.
Modules
=============
How do I create a .pyc file?
-------------------------------------
When a module is imported for the first time (or when the source is
more recent than the current compiled file) a ``.pyc`` file containing
the compiled code should be created in the same directory as the
``.py`` file.
One reason that a ``.pyc`` file may not be created is permissions
problems with the directory. This can happen, for example, if you
develop as one user but run as another, such as if you are testing
with a web server. Creation of a .pyc file is automatic if you're
importing a module and Python has the ability (permissions, free
space, etc...) to write the compiled module back to the directory.
Running Python on a top level script is not considered an import and
no ``.pyc`` will be created. For example, if you have a top-level
module ``abc.py`` that imports another module ``xyz.py``, when you run
abc, ``xyz.pyc`` will be created since xyz is imported, but no
``abc.pyc`` file will be created since ``abc.py`` isn't being
imported.
If you need to create abc.pyc -- that is, to create a .pyc file for a
module that is not imported -- you can, using the py_compile and
compileall modules.
The ``py_compile`` module can manually compile any module. One way is
to use the ``compile()`` function in that module interactively::
>>> import py_compile
>>> py_compile.compile('abc.py')
This will write the ``.pyc`` to the same location as ``abc.py`` (or
you can override that with the optional parameter ``cfile``).
You can also automatically compile all files in a directory or
directories using the ``compileall`` module.
You can do it from the shell prompt by running ``compileall.py``
and providing the path of a directory containing Python files to compile::
python compileall.py .
How do I find the current module name?
---------------------------------------------
A module can find out its own module name by looking at the predefined
global variable ``__name__``. If this has the value '__main__', the
program is running as a script. Many modules that are usually used by
importing them also provide a command-line interface or a self-test,
and only execute this code after checking ``__name__``::
def main():
print 'Running test...'
...
if __name__ == '__main__':
main()
How can I have modules that mutually import each other?
---------------------------------------------------------------
Suppose you have the following modules:
foo.py::
from bar import bar_var
foo_var=1
bar.py::
from foo import foo_var
bar_var=2
The problem is that the interpreter will perform the following steps:
* main imports foo
* Empty globals for foo are created
* foo is compiled and starts executing
* foo imports bar
* Empty globals for bar are created
* bar is compiled and starts executing
* bar imports foo (which is a no-op since there already is a module named foo)
* bar.foo_var = foo.foo_var
The last step fails, because Python isn't done with interpreting ``foo``
yet and the global symbol dictionary for ``foo`` is still empty.
The same thing happens when you use ``import foo``, and then try to
access ``foo.foo_var`` in global code.
There are (at least) three possible workarounds for this problem.
Guido van Rossum recommends avoiding all uses of ``from <module>
import ...``, and placing all code inside functions. Initializations
of global variables and class variables should use constants or
built-in functions only. This means everything from an imported
module is referenced as ``<module>.<name>``.
Jim Roskind suggests performing steps in the following order in each
module:
* exports (globals, functions, and classes that don't need imported base classes)
* ``import`` statements
* active code (including globals that are initialized from imported values).
van Rossum doesn't like this approach much because the imports
appear in a strange place, but it does work.
Matthias Urlichs recommends restructuring your code so that the
recursive import is not necessary in the first place.
These solutions are not mutually exclusive.
__import__('x.y.z') returns <module 'x'>; how do I get z?
-----------------------------------------------------------------------
Try::
__import__('x.y.z').y.z
For more realistic situations, you may have to do something like ::
m = __import__(s)
for i in s.split(".")[1:]:
m = getattr(m, i)
When I edit an imported module and reimport it, the changes don't show up. Why does this happen?
--------------------------------------------------------------------------------------------------------------------------------------------
For reasons of efficiency as well as consistency, Python only reads
the module file on the first time a module is imported. If it didn't,
in a program consisting of many modules where each one imports the
same basic module, the basic module would be parsed and re-parsed many
times. To force rereading of a changed module, do this::
import modname
reload(modname)
Warning: this technique is not 100% fool-proof. In particular,
modules containing statements like ::
from modname import some_objects
will continue to work with the old version of the imported objects.
If the module contains class definitions, existing class instances
will *not* be updated to use the new class definition. This can
result in the following paradoxical behaviour::
>>> import cls
>>> c = cls.C() # Create an instance of C
>>> reload(cls)
<module 'cls' from 'cls.pyc'>
>>> isinstance(c, cls.C) # isinstance is false?!?
False
The nature of the problem is made clear if you print out the class objects:
>>> c.__class__
<class cls.C at 0x7352a0>
>>> cls.C
<class cls.C at 0x4198d0>