Python 3: How to check if a string can be a valid variable?

Question

I have a string and want to check if it can be used as a valid variable without getting a syntax error. For example

def variableName(string):
    #if string is valid variable name:
        #return True
    #else:
        #return False

input >>> variableName("validVariable")
output >>> True
input >>> variableName("992variable")
output >>> False

I would not like to use the .isidentifier(). I want to make a function of my own.

Answer 1

The following answer is true only for "old-style" Python-2.7 identifiers;

"validVariable".isidentifier()
#True
"992variable".isidentifier()
#False

Since you changed your question after I posted the answer, consider writing a regular expression:

re.match(r"[_a-z]\w*$", yourstring,flags=re.I)

Answer 2

In Python 3 a valid identifier can have characters outside of ASCII range, as you don't want to use str.isidentifier , you can write your own version of it in Python.

Its specification can be found here: https://www.python.org/dev/peps/pep-3131/#specification-of-language-changes

Implementation:

import keyword
import re
import unicodedata


def is_other_id_start(char):
    """
    Item belongs to Other_ID_Start in
    http://unicode.org/Public/UNIDATA/PropList.txt
    """
    return bool(re.match(r'[\u1885-\u1886\u2118\u212E\u309B-\u309C]', char))


def is_other_id_continue(char):
    """
    Item belongs to Other_ID_Continue in
    http://unicode.org/Public/UNIDATA/PropList.txt
    """
    return bool(re.match(r'[\u00B7\u0387\u1369-\u1371\u19DA]', char))


def is_xid_start(char):

    # ID_Start is defined as all characters having one of
    # the general categories uppercase letters(Lu), lowercase
    # letters(Ll), titlecase letters(Lt), modifier letters(Lm),
    # other letters(Lo), letter numbers(Nl), the underscore, and
    # characters carrying the Other_ID_Start property. XID_Start
    # then closes this set under normalization, by removing all
    # characters whose NFKC normalization is not of the form
    # ID_Start ID_Continue * anymore.

    category = unicodedata.category(char)
    return (
        category in {'Lu', 'Ll', 'Lt', 'Lm', 'Lo', 'Nl'} or
        is_other_id_start(char)
    )


def is_xid_continue(char):
    # ID_Continue is defined as all characters in ID_Start, plus
    # nonspacing marks (Mn), spacing combining marks (Mc), decimal
    # number (Nd), connector punctuations (Pc), and characters
    # carryig the Other_ID_Continue property. Again, XID_Continue
    # closes this set under NFKC-normalization; it also adds U+00B7
    # to support Catalan.

    category = unicodedata.category(char)
    return (
        is_xid_start(char) or
        category in {'Mn', 'Mc', 'Nd', 'Pc'} or
        is_other_id_continue(char)
    )


def is_valid_identifier(name):
    # All identifiers are converted into the normal form NFKC
    # while parsing; comparison of identifiers is based on NFKC.
    name = unicodedata.normalize(
        'NFKC', name
    )

    # check if it's a keyword
    if keyword.iskeyword(name):
        return False

    # The identifier syntax is <XID_Start> <XID_Continue>*.
    if not (is_xid_start(name[0]) or name[0] == '_'):
        return False

    return all(is_xid_continue(char) for char in name[1:])

if __name__ == '__main__':
    # From goo.gl/pvpYg6
    assert is_valid_identifier("a") is True
    assert is_valid_identifier("Z") is True
    assert is_valid_identifier("_") is True
    assert is_valid_identifier("b0") is True
    assert is_valid_identifier("bc") is True
    assert is_valid_identifier("b_") is True
    assert is_valid_identifier("µ") is True
    assert is_valid_identifier("𝔘𝔫𝔦𝔠𝔬𝔡𝔢") is True

    assert is_valid_identifier(" ") is False
    assert is_valid_identifier("[") is False
    assert is_valid_identifier("©") is False
    assert is_valid_identifier("0") is False

---

You can check CPython and Pypy's implmentation here and here respectively.

Answer 3

You could use a regular expression.

For example:

isValidIdentifier = re.match("[A-Za-z_](0-9A-Za-z_)*",identifier)

Note that his only checks for alphanumeric characters. The actual standard supports other characters. See here: https://www.python.org/dev/peps/pep-3131/

You may also need to exclude reserved words such as def, True, False, ... see here: https://www.programiz.com/python-programming/keywords-identifier