Convert camel-case to snake-case in python.

camel_case_to_snake_case.py

#!/usr/bin/env python

"""
Convert camel-case to snake-case in python.

e.g.: CamelCase -> snake_case

Relevant StackOverflow question: http://stackoverflow.com/a/1176023/293064
"""

__author__ = 'Jay Taylor [@jtaylor]'


import re


_underscorer1 = re.compile(r'(.)([A-Z][a-z]+)')
_underscorer2 = re.compile('([a-z0-9])([A-Z])')

def camelToSnake(s):
    """ 
    Is it ironic that this function is written in camel case, yet it
    converts to snake case? hmm..
    """
    subbed = _underscorer1.sub(r'\1_\2', s)
    return _underscorer2.sub(r'\1_\2', subbed).lower()

if __name__ == '__main__':
    assert camelToSnake('snakesOnAPlane') == 'snakes_on_a_plane'
    assert camelToSnake('SnakesOnAPlane') == 'snakes_on_a_plane'
    assert camelToSnake('snakes_on_a_plane') == 'snakes_on_a_plane'
    assert camelToSnake('IPhoneHysteria') == 'i_phone_hysteria'
    assert camelToSnake('iPhoneHysteria') == 'i_phone_hysteria'
    print 'All tests passed.'

My variant of @dopstar's solution, adding an additional test to handle all-caps values.

LEADING_UC_CAMEL_SEGMENT = re.compile(r'([A-Z]+)([A-Z][a-z])')
UC_CAMEL_SEGMENT = re.compile(r'([a-z\d])([A-Z])')


def camel_to_snake(value: str) -> str:
    """
    Convert a CamelCasedString to a snake_cased_string. Separates capitalized words with underscores,
    and lower-cases the result. Runs of multiple uppercase letters are not split, e.g.
    "HTTPPort" would become "http_port".

    :param value: a string value
    :return: the converted value
    """
    lower = value.lower()
    if value.upper() == lower:  # value is all caps
        return lower
    else:
        return UC_CAMEL_SEGMENT.sub(r'\1_\2',
                                    LEADING_UC_CAMEL_SEGMENT.sub(r'\1_\2', value)).lower()

I tried lots of things which didn't support all cases and I made this one

def to_snake(camel_input):
        camel_input = camel_input.replace('&', '_and_')
        camel_input = re.sub(r'(?<!^)(?=[A-Z])', '_', camel_input).lower()
        words = re.findall(r'[A-Z]?[a-z]+|[A-Z]{2,}(?=[A-Z][a-z]|\d|\W|$)|\d+', camel_input)
        return '_'.join(words)

result for "'my p&l AndP&L aa_BB_dd (teSt) AndMy P&L andAlsoP&L andAlso p&l A B aA'":

print(to_snake('my p&l AndP&L aa_BB_dd (teSt) AndMy P&L andAlsoP&L andAlso p&l A B aA'))

my_p_and_l_and_p_and_l_aa_b_b_dd_te_st_and_my_p_and_l_and_also_p_and_l_and_also_p_and_l_a_b_a_a