Pythonically check if a variable name is valid

Question

tldr; see the final line; the rest is just preamble.

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I am developing a test harness, which parses user scripts and generates a Python script which it then runs. The idea is for non-techie folks to be able to write high-level test scripts.

I have introduced the idea of variables, so a user can use the LET keyword in his script. E.g. LET X = 42, which I simply expand to X = 42. They can then use X later in their scripts - RELEASE CONNECTION X

But what if someone writes LET 2 = 3? That's going to generate invalid Python.

If I have that X in a variable variableName, then how can I check whether variableName is a valid Python variable?

Answer 1

In Python 3 you can use str.isidentifier() to test whether a given string is a valid Python identifier/name.

>>> 'X'.isidentifier()
True
>>> 'X123'.isidentifier()
True
>>> '2'.isidentifier()
False
>>> 'while'.isidentifier()
True

The last example shows that you should also check whether the variable name clashes with a Python keyword:

>>> from keyword import iskeyword
>>> iskeyword('X')
False
>>> iskeyword('while')
True

So you could put that together in a function:

from keyword import iskeyword

def is_valid_variable_name(name):
    return name.isidentifier() and not iskeyword(name)

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Another option, which works in Python 2 and 3, is to use the ast module:

from ast import parse

def is_valid_variable_name(name):
    try:
        parse('{} = None'.format(name))
        return True
    except SyntaxError, ValueError, TypeError:
        return False

>>> is_valid_variable_name('X')
True
>>> is_valid_variable_name('123')
False
>>> is_valid_variable_name('for')
False
>>> is_valid_variable_name('')
False
>>> is_valid_variable_name(42)
False

This will parse the assignment statement without actually executing it. It will pick up invalid identifiers as well as attempts to assign to a keyword. In the above code None is an arbitrary value to assign to the given name - it could be any valid expression for the RHS.

Answer 2

EDIT: this is wrong and implementation dependent - see comments.

Just have Python do its own check by making a dictionary with the variable holding the name as the key and splatting it as keyword arguments:

def _dummy_function(**kwargs):
    pass

def is_valid_variable_name(name):
    try:
        _dummy_function(**{name: None})
        return True
    except TypeError:
        return False

Notably, TypeError is consistently raised whenever a dict splats into keyword arguments but has a key which isn't a valid function argument, and whenever a dict literal is being constructed with an invalid key, so this will work correctly on anything you pass to it.