lisper learning python, ... could you please comment my first python program?

[neptundancer at gmail.com]

Thanks a lot for comments! I am going to fix the source according to
your advices ;)

Nep

On Aug 26, 6:32 pm, Steven D'Aprano <st... at REMOVE-
THIScybersource.com.au> wrote:
> On Sun, 26 Aug 2007 15:56:08 +0000, neptundancer wrote:
> > Hi,
> >   to extend my skills, I am learning python. I have written small
> > program which computes math expression like "1+2*sin(y^10)/cos(x*y)" and
> > similar, so far only + - * / ^ sin con tan sqrt are supported. But my
> > program is quite inextensible, I have to change the code to add new
> > functions... Could some fellow experienced pythonista give me some tips
> > how to make my program shorter, and more extensible?
>
> Just a few comments at random. This is certainly not meant to be
> exhaustive:
>
> > def normalize(string):
> >     tmp = "".join([c for c in string if c != " "])
> >     return "(" + tmp + ")"
>
> Try this instead:
>
> def normalize(astring):
>     return "(" + astring.replace(" ", "") + ")"
>
>
>
> > def most_nested_expression(string):
> [snip code]
> >     if level != 0:
> >         raise IOError("mismatched parens")
>
> You raise IOError quite often, but that's a misuse of it. IOError is a
> subclass of EnvironmentError, and is meant to indicate (e.g.) a failed
> read from a disk.
>
> If you execute help(IOError) at the interactive prompt, you will see:
>
> class IOError(EnvironmentError)
>  |  I/O operation failed.
>
> I suggest a better exception to use would be ValueError, or even create
> your own:
>
> class MyCustomError(ValueError):
>     pass
>
> would be a minimal example.
>
> > def is_reduced_expression(string):
> >     for c in string:
> >         if c == "^" or c == "*" or c == "/" or c == "+" or c == "-":
> >             return False
> >     return True
>
> Change that to:
>
> for c in string:
>     if c in "^*/+-": return False
> return True
>
> [snip]
>
> > def add(a, b): return a + b
> > def sub(a, b): return a - b
> > def mul(a, b): return a * b
> > def div(a, b): return a / b
>
> Replace the above four functions with:
>
> from operator import add, sub, mul
> from operator import truediv as div
>
> > def translate_function(fn_str):
> >     if fn_str == "+": return add
> >     elif fn_str == "-": return sub
> >     elif fn_str == "*": return mul
> >     elif fn_str == "/": return div
> >     elif fn_str == "^": return math.pow
> >     elif fn_str == "sin": return math.sin elif fn_str == "cos": return
> >     math.cos elif fn_str == "tan": return math.tan elif fn_str ==
> >     "sqrt": return math.sqrt else: raise IOError("unknown function %s" %
> >     fn_str)
>
> fn_map = {"+": add, "-": sub, "*": mul, "/": div,
>     "^": math.pow, "sin": math.sin } # etc.
>
> def translate_function(fn_str):
>     try:
>         return fn_map[fn_str]
>     except KeyError:
>         raise ValueError("unknown function '%s'" % fn_str)
>
> Hope that helps.
>
> --
> Steven.