Elegant Python function to convert CamelCase to snake_case?

Camel case to snake case

import re

name = 'CamelCaseName'
name = re.sub(r'(?<!^)(?=[A-Z])', '_', name).lower()
print(name)  # camel_case_name

If you do this many times and the above is slow, compile the regex beforehand:

pattern = re.compile(r'(?<!^)(?=[A-Z])')
name = pattern.sub('_', name).lower()

Note that this and immediately following regex use a zero-width match, which is not handled correctly by Python 3.6 or earlier. See further below for alternatives that don't use lookahead/lookbehind if you need to support older EOL Python.

If you want to avoid converting "HTTPHeader" into "h_t_t_p_header", you can use this variant with regex alternation:

pattern = re.compile(r"(?<=[a-z])(?=[A-Z])|(?<=[A-Z])(?=[A-Z][a-z])")
name = pattern.sub('_', name).lower()

See Regex101.com for test cases (that don't include final lowercase).

You can improve readability with ?x or re.X:

pattern = re.compile(
    r"""
        (?<=[a-z])      # preceded by lowercase
        (?=[A-Z])       # followed by uppercase
        |               #   OR
        (?<[A-Z])       # preceded by lowercase
        (?=[A-Z][a-z])  # followed by uppercase, then lowercase
    """,
    re.X,
)

If you use the regex module instead of re, you can use the more readable POSIX character classes (which are not limited to ASCII).

pattern = re.compile(
    r"""
        (?<=[[:lower:]])            # preceded by lowercase
        (?=[[:upper:]])             # followed by uppercase
        |                           #   OR
        (?<[[:upper:]])             # preceded by lower
        (?=[[:upper:]][[:lower:]])  # followed by upper then lower
    """,
    re.X,
)

Another way to handle more advanced cases without relying on lookahead/lookbehind, using two substitution passes:

def camel_to_snake(name):
    name = re.sub('(.)([A-Z][a-z]+)', r'\1_\2', name)
    return re.sub('([a-z0-9])([A-Z])', r'\1_\2', name).lower()

print(camel_to_snake('camel2_camel2_case'))  # camel2_camel2_case
print(camel_to_snake('getHTTPResponseCode'))  # get_http_response_code
print(camel_to_snake('HTTPResponseCodeXYZ'))  # http_response_code_xyz

To add also cases with two underscores or more:

def to_snake_case(name):
    name = re.sub('(.)([A-Z][a-z]+)', r'\1_\2', name)
    name = re.sub('__([A-Z])', r'_\1', name)
    name = re.sub('([a-z0-9])([A-Z])', r'\1_\2', name)
    return name.lower()

Snake case to pascal case

name = 'snake_case_name'
name = ''.join(word.title() for word in name.split('_'))
print(name)  # SnakeCaseName

There's an inflection library in the package index that can handle these things for you. In this case, you'd be looking for inflection.underscore():

>>> inflection.underscore('CamelCase')
'camel_case'

I don't know why these are all so complicating.

for most cases, the simple expression ([A-Z]+) will do the trick

>>> re.sub('([A-Z]+)', r'_\1','CamelCase').lower()
'_camel_case'  
>>> re.sub('([A-Z]+)', r'_\1','camelCase').lower()
'camel_case'
>>> re.sub('([A-Z]+)', r'_\1','camel2Case2').lower()
'camel2_case2'
>>> re.sub('([A-Z]+)', r'_\1','camelCamelCase').lower()
'camel_camel_case'
>>> re.sub('([A-Z]+)', r'_\1','getHTTPResponseCode').lower()
'get_httpresponse_code'

To ignore the first character simply add look behind (?!^)

>>> re.sub('(?!^)([A-Z]+)', r'_\1','CamelCase').lower()
'camel_case'
>>> re.sub('(?!^)([A-Z]+)', r'_\1','CamelCamelCase').lower()
'camel_camel_case'
>>> re.sub('(?!^)([A-Z]+)', r'_\1','Camel2Camel2Case').lower()
'camel2_camel2_case'
>>> re.sub('(?!^)([A-Z]+)', r'_\1','getHTTPResponseCode').lower()
'get_httpresponse_code'

If you want to separate ALLCaps to all_caps and expect numbers in your string you still don't need to do two separate runs just use | This expression ((?<=[a-z0-9])[A-Z]|(?!^)[A-Z](?=[a-z])) can handle just about every scenario in the book

>>> a = re.compile('((?<=[a-z0-9])[A-Z]|(?!^)[A-Z](?=[a-z]))')
>>> a.sub(r'_\1', 'getHTTPResponseCode').lower()
'get_http_response_code'
>>> a.sub(r'_\1', 'get2HTTPResponseCode').lower()
'get2_http_response_code'
>>> a.sub(r'_\1', 'get2HTTPResponse123Code').lower()
'get2_http_response123_code'
>>> a.sub(r'_\1', 'HTTPResponseCode').lower()
'http_response_code'
>>> a.sub(r'_\1', 'HTTPResponseCodeXYZ').lower()
'http_response_code_xyz'

It all depends on what you want so use the solution that best suits your needs as it should not be overly complicated.

nJoy!

stringcase is my go-to library for this; e.g.:

>>> from stringcase import pascalcase, snakecase
>>> snakecase('FooBarBaz')
'foo_bar_baz'
>>> pascalcase('foo_bar_baz')
'FooBarBaz'