[Python-3000] PEP 3101 update
Talin
talin at acm.org
Sun Jun 11 03:07:05 CEST 2006
Here's the latest PEP 3101 - I've incorporated changes based on
suggestions from a lot of folks. This version incorporates:
-- a detailed specification for conversion type fields
-- description of error handling behavior
-- 'strict' vs. 'lenient' error handling flag
-- compound field names
-- braces are now escaped using {{ instead of \{
--------------------------------------------------------------------
PEP: 3101
Title: Advanced String Formatting
Version: $Revision: 46845 $
Last-Modified: $Date: 2006-06-10 17:59:06 -0700 (Sat, 10 Jun 2006) $
Author: Talin <talin at acm.org>
Status: Draft
Type: Standards
Content-Type: text/plain
Created: 16-Apr-2006
Python-Version: 3.0
Post-History: 28-Apr-2006, 6-May-2006, 10-Jun-2006
Abstract
This PEP proposes a new system for built-in string formatting
operations, intended as a replacement for the existing '%' string
formatting operator.
Rationale
Python currently provides two methods of string interpolation:
- The '%' operator for strings. [1]
- The string.Template module. [2]
The scope of this PEP will be restricted to proposals for built-in
string formatting operations (in other words, methods of the
built-in string type).
The '%' operator is primarily limited by the fact that it is a
binary operator, and therefore can take at most two arguments.
One of those arguments is already dedicated to the format string,
leaving all other variables to be squeezed into the remaining
argument. The current practice is to use either a dictionary or a
tuple as the second argument, but as many people have commented
[3], this lacks flexibility. The "all or nothing" approach
(meaning that one must choose between only positional arguments,
or only named arguments) is felt to be overly constraining.
While there is some overlap between this proposal and
string.Template, it is felt that each serves a distinct need,
and that one does not obviate the other. In any case,
string.Template will not be discussed here.
Specification
The specification will consist of the following parts:
- Specification of a new formatting method to be added to the
built-in string class.
- Specification of a new syntax for format strings.
- Specification of a new set of class methods to control the
formatting and conversion of objects.
- Specification of an API for user-defined formatting classes.
- Specification of how formatting errors are handled.
Note on string encodings: Since this PEP is being targeted
at Python 3.0, it is assumed that all strings are unicode strings,
and that the use of the word 'string' in the context of this
document will generally refer to a Python 3.0 string, which is
the same as Python 2.x unicode object.
If it should happen that this functionality is backported to
the 2.x series, then it will be necessary to handle both regular
string as well as unicode objects. All of the function call
interfaces described in this PEP can be used for both strings
and unicode objects, and in all cases there is sufficient
information to be able to properly deduce the output string
type (in other words, there is no need for two separate APIs).
In all cases, the type of the template string dominates - that
is, the result of the conversion will always result in an object
that contains the same representation of characters as the
input template string.
String Methods
The build-in string class will gain a new method, 'format',
which takes takes an arbitrary number of positional and keyword
arguments:
"The story of {0}, {1}, and {c}".format(a, b, c=d)
Within a format string, each positional argument is identified
with a number, starting from zero, so in the above example, 'a' is
argument 0 and 'b' is argument 1. Each keyword argument is
identified by its keyword name, so in the above example, 'c' is
used to refer to the third argument.
Format Strings
Brace characters ('curly braces') are used to indicate a
replacement field within the string:
"My name is {0}".format('Fred')
The result of this is the string:
"My name is Fred"
Braces can be escaped by doubling:
"My name is {0} :-{{}}".format('Fred')
Which would produce:
"My name is Fred :-{}"
The element within the braces is called a 'field'. Fields consist
of a 'field name', which can either be simple or compound, and an
optional 'conversion specifier'.
Simple and Compound Field Names
Simple field names are either names or numbers. If numbers, they
must be valid base-10 integers; if names, they must be valid
Python identifiers. A number is used to identify a positional
argument, while a name is used to identify a keyword argument.
A compound field name is a combination of multiple simple field
names in an expression:
"My name is {0.name}".format(file('out.txt'))
This example shows the use of the 'getattr' or 'dot' operator
in a field expression. The dot operator allows an attribute of
an input value to be specified as the field value.
The types of expressions that can be used in a compound name
have been deliberately limited in order to prevent potential
security exploits resulting from the ability to place arbitrary
Python expressions inside of strings. Only two operators are
supported, the '.' (getattr) operator, and the '[]' (getitem)
operator.
An example of the 'getitem' syntax:
"My name is {0[name]}".format(dict(name='Fred'))
It should be noted that the use of 'getitem' within a string is
much more limited than its normal use. In the above example, the
string 'name' really is the literal string 'name', not a variable
named 'name'. The rules for parsing an item key are the same as
for parsing a simple name - in other words, if it looks like a
number, then its treated as a number, if it looks like an
identifier, then it is used as a string.
It is not possible to specify arbitrary dictionary keys from
within a format string.
Conversion Specifiers
Each field can also specify an optional set of 'conversion
specifiers' which can be used to adjust the format of that field.
Conversion specifiers follow the field name, with a colon (':')
character separating the two:
"My name is {0:8}".format('Fred')
The meaning and syntax of the conversion specifiers depends on the
type of object that is being formatted, however many of the
built-in types will recognize a standard set of conversion
specifiers.
Conversion specifiers can themselves contain replacement fields.
For example, a field whose field width it itself a parameter
could be specified via:
"{0:{1}}".format(a, b, c)
Note that the doubled '}' at the end, which would normally be
escaped, is not escaped in this case. The reason is because
the '{{' and '}}' syntax for escapes is only applied when used
*outside* of a format field. Within a format field, the brace
characters always have their normal meaning.
The syntax for conversion specifiers is open-ended, since except
than doing field replacements, the format() method does not
attempt to interpret them in any way; it merely passes all of the
characters between the first colon and the matching brace to
the various underlying formatter methods.
Standard Conversion Specifiers
If an object does not define its own conversion specifiers, a
standard set of conversion specifiers are used. These are similar
in concept to the conversion specifiers used by the existing '%'
operator, however there are also a number of significant
differences. The standard conversion specifiers fall into three
major categories: string conversions, integer conversions and
floating point conversions.
The general form of a standard conversion specifier is:
[[fill]align][sign][width][.precision][type]
The brackets ([]) indicate an optional field.
Then the optional align flag can be one of the following:
'<' - Forces the field to be left-aligned within the available
space (This is the default.)
'>' - Forces the field to be right-aligned within the
available space.
'=' - Forces the padding to be placed between immediately
after the sign, if any. This is used for printing fields
in the form '+000000120'.
Note that unless a minimum field width is defined, the field
width will always be the same size as the data to fill it, so
that the alignment option has no meaning in this case.
The optional 'fill' character defines the character to be used to
pad the field to the minimum width. The alignment flag must be
supplied if the character is a number other than 0 (otherwise the
character would be interpreted as part of the field width
specifier). A zero fill character without an alignment flag
implies an alignment type of '='.
The 'sign' field can be one of the following:
'+' - indicates that a sign should be used for both
positive as well as negative numbers
'-' - indicates that a sign should be used only for negative
numbers (this is the default behaviour)
' ' - indicates that a leading space should be used on
positive numbers
'()' - indicates that negative numbers should be surrounded
by parentheses
'width' is a decimal integer defining the minimum field width. If
not specified, then the field width will be determined by the
content.
The 'precision' field is a decimal number indicating how many
digits should be displayed after the decimal point.
Finally, the 'type' determines how the data should be presented.
If the type field is absent, an appropriate type will be assigned
based on the value to be formatted ('d' for integers and longs,
'g' for floats, and 's' for everything else.)
The available string conversion types are:
's' - String format. Invokes str() on the object.
This is the default conversion specifier type.
'r' - Repr format. Invokes repr() on the object.
There are several integer conversion types. All invoke int() on
the object before attempting to format it.
The available integer conversion types are:
'b' - Binary. Outputs the number in base 2.
'c' - Character. Converts the integer to the corresponding
unicode character before printing.
'd' - Decimal Integer. Outputs the number in base 10.
'o' - Octal format. Outputs the number in base 8.
'x' - Hex format. Outputs the number in base 16, using lower-
case letters for the digits above 9.
'X' - Hex format. Outputs the number in base 16, using upper-
case letters for the digits above 9.
There are several floating point conversion types. All invoke
float() on the object before attempting to format it.
The available floating point conversion types are:
'e' - Exponent notation. Prints the number in scientific
notation using the letter 'e' to indicate the exponent.
'E' - Exponent notation. Same as 'e' except it uses an upper
case 'E' as the separator character.
'f' - Fixed point. Displays the number as a fixed-point
number.
'F' - Fixed point. Same as 'f'.
'g' - General format. This prints the number as a fixed-point
number, unless the number is too large, in which case
it switches to 'e' exponent notation.
'G' - General format. Same as 'g' except switches to 'E'
if the number gets to large.
'n' - Number. This is the same as 'g', except that it uses the
current locale setting to insert the appropriate
number separator characters.
'%' - Percentage. Multiplies the number by 100 and displays
in fixed ('f') format, followed by a percent sign.
Objects are able to define their own conversion specifiers to
replace the standard ones. An example is the 'datetime' class,
whose conversion specifiers might look something like the
arguments to the strftime() function:
"Today is: {0:a b d H:M:S Y}".format(datetime.now())
Controlling Formatting
A class that wishes to implement a custom interpretation of its
conversion specifiers can implement a __format__ method:
class AST:
def __format__(self, specifiers):
...
The 'specifiers' argument will be either a string object or a
unicode object, depending on the type of the original format
string. The __format__ method should test the type of the
specifiers parameter to determine whether to return a string or
unicode object. It is the responsibility of the __format__ method
to return an object of the proper type.
string.format() will format each field using the following steps:
1) See if the value to be formatted has a __format__ method. If
it does, then call it.
2) Otherwise, check the internal formatter within string.format
that contains knowledge of certain builtin types.
3) Otherwise, call str() or unicode() as appropriate.
User-Defined Formatting Classes
There will be times when customizing the formatting of fields
on a per-type basis is not enough. An example might be an
accounting application, which displays negative numbers in
parentheses rather than using a negative sign.
The string formatting system facilitates this kind of application-
specific formatting by allowing user code to directly invoke
the code that interprets format strings and fields. User-written
code can intercept the normal formatting operations on a per-field
basis, substituting their own formatting methods.
For example, in the aforementioned accounting application, there
could be an application-specific number formatter, which reuses
the string.format templating code to do most of the work. The
API for such an application-specific formatter is up to the
application; here are several possible examples:
cell_format("The total is: {0}", total)
TemplateString("The total is: {0}").format(total)
Creating an application-specific formatter is relatively straight-
forward. The string and unicode classes will have a class method
called 'cformat' that does all the actual work of formatting; The
built-in format() method is just a wrapper that calls cformat.
The type signature for the cFormat function is as follows:
cformat(template, format_hook, args, kwargs)
The parameters to the cformat function are:
-- The format template string.
-- A callable 'format hook', which is called once per field
-- A tuple containing the positional arguments
-- A dict containing the keyword arguments
The cformat function will parse all of the fields in the format
string, and return a new string (or unicode) with all of the
fields replaced with their formatted values.
The format hook is a callable object supplied by the user, which
is invoked once per field, and which can override the normal
formatting for that field. For each field, the cformat function
will attempt to call the field format hook with the following
arguments:
format_hook(value, conversion)
The 'value' field corresponds to the value being formatted, which
was retrieved from the arguments using the field name.
The 'conversion' argument is the conversion spec part of the
field, which will be either a string or unicode object, depending
on the type of the original format string.
The field_hook will be called once per field. The field_hook may
take one of two actions:
1) Return a string or unicode object that is the result
of the formatting operation.
2) Return None, indicating that the field_hook will not
process this field and the default formatting should be
used. This decision should be based on the type of the
value object, and the contents of the conversion string.
Error handling
The string formatting system has two error handling modes, which
are controlled by the value of a class variable:
string.strict_format_errors = True
The 'strict_format_errors' flag defaults to False, or 'lenient'
mode. Setting it to True enables 'strict' mode. The current mode
determines how errors are handled, depending on the type of the
error.
The types of errors that can occur are:
1) Reference to a missing or invalid argument from within a
field specifier. In strict mode, this will raise an exception.
In lenient mode, this will cause the value of the field to be
replaced with the string '?name?', where 'name' will be the
type of error (KeyError, IndexError, or AttributeError).
So for example:
>>> string.strict_format_errors = False
>>> print 'Item 2 of argument 0 is: {0[2]}'.format( [0,1] )
"Item 2 of argument 0 is: ?IndexError?"
2) Unused argument. In strict mode, this will raise an exception.
In lenient mode, this will be ignored.
3) Exception raised by underlying formatter. These exceptions
are always passed through, regardless of the current mode.
Alternate Syntax
Naturally, one of the most contentious issues is the syntax of the
format strings, and in particular the markup conventions used to
indicate fields.
Rather than attempting to exhaustively list all of the various
proposals, I will cover the ones that are most widely used
already.
- Shell variable syntax: $name and $(name) (or in some variants,
${name}). This is probably the oldest convention out there, and
is used by Perl and many others. When used without the braces,
the length of the variable is determined by lexically scanning
until an invalid character is found.
This scheme is generally used in cases where interpolation is
implicit - that is, in environments where any string can contain
interpolation variables, and no special subsitution function
need be invoked. In such cases, it is important to prevent the
interpolation behavior from occuring accidentally, so the '$'
(which is otherwise a relatively uncommonly-used character) is
used to signal when the behavior should occur.
It is the author's opinion, however, that in cases where the
formatting is explicitly invoked, that less care needs to be
taken to prevent accidental interpolation, in which case a
lighter and less unwieldy syntax can be used.
- Printf and its cousins ('%'), including variations that add a
field index, so that fields can be interpolated out of order.
- Other bracket-only variations. Various MUDs (Multi-User
Dungeons) such as MUSH have used brackets (e.g. [name]) to do
string interpolation. The Microsoft .Net libraries uses braces
({}), and a syntax which is very similar to the one in this
proposal, although the syntax for conversion specifiers is quite
different. [4]
- Backquoting. This method has the benefit of minimal syntactical
clutter, however it lacks many of the benefits of a function
call syntax (such as complex expression arguments, custom
formatters, etc.).
- Other variations include Ruby's #{}, PHP's {$name}, and so
on.
Some specific aspects of the syntax warrant additional comments:
1) Backslash character for escapes. The original version of
this PEP used backslash rather than doubling to escape a bracket.
This worked because backslashes in Python string literals that
don't conform to a standard backslash sequence such as '\n'
are left unmodified. However, this caused a certain amount
of confusion, and led to potential situations of multiple
recursive escapes, i.e. '\\\\{' to place a literal backslash
in front of a bracket.
2) The use of the colon character (':') as a separator for
conversion specifiers. This was chosen simply because that's
what .Net uses.
Sample Implementation
A rough prototype of the underlying 'cformat' function has been
coded in Python, however it needs much refinement before being
submitted.
Backwards Compatibility
Backwards compatibility can be maintained by leaving the existing
mechanisms in place. The new system does not collide with any of
the method names of the existing string formatting techniques, so
both systems can co-exist until it comes time to deprecate the
older system.
References
[1] Python Library Reference - String formating operations
http://docs.python.org/lib/typesseq-strings.html
[2] Python Library References - Template strings
http://docs.python.org/lib/node109.html
[3] [Python-3000] String formating operations in python 3k
http://mail.python.org/pipermail/python-3000/2006-April/000285.html
[4] Composite Formatting - [.Net Framework Developer's Guide]
http://msdn.microsoft.com/library/en-us/cpguide/html/cpconcompositeformatting.asp?frame=true
Copyright
This document has been placed in the public domain.
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