Elegant Python function to convert CamelCase to snake_case?

Example:

>>> convert('CamelCase')
'camel_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