Why does python disallow usage of hyphens within function and variable names?

I have always wondered why can't we use hyphens in between function names and variable names in python

H开发者_开发知识库aving tried functional programming languages like Lisp and Clojure, where hyphens are allowed. Why python doesn't do that.

# This won't work -- SyntaxError
def is-even(num):
    return num % 2

# This will work
def is_even(num):
    return num % 2

I am sure Sir Guido must have done this because of some reasons. I googled but couldn't manage to find the answer. Can anyone please throw some light on this?

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Because hyphen is used as the subtraction operator. Imagine that you could have an is-even function, and then you had code like this:

my_var = is-even(another_var)

Is is-even(another_var) a call to the function is-even, or is it subtracting the result of the function even from a variable named is?

Lisp dialects don't have this problem, since they use prefix notation. For example, there's clear difference between

(is-even 4)

and

(- is (even 4))

in Lisps.

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Because Python uses infix notation to represent calculations and a hyphen and a minus has the exact same ascii code. You can have ambiguous cases such as:

a-b = 10
a = 1
b = 1

c = a-b

What is the answer? 0 or 10?

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Because it would make the parser even more complicated. It would be confusing too for the programmers.

Consider def is-even(num): : now, if is is a global variable, what happens?

Also note that the - is the subtraction operator in Python, hence would further complicate parsing.

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is-even(num)

contains a hyphen ? I thought it was a subtraction of the value returned by function even with argument num from the value of is.

As @jdupont says, parsing can be tricky.

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Oddly enough it is possible to have class variable names with hyphens using setattr(), not that you would want to. Here is an example:

class testclass:
    pass

x = testclass()
setattr(x, "is-even", True)
getattr(x, "is-even")
True

This still fails:

x.is-even
File "<stdin>", line 1
  x.is-even
     ^
SyntaxError: invalid syntax