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.
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