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()

To handle more advanced cases specially (this is not reversible anymore):

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 camel case

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

I think this solution is more straightforward than previous answers:

import re

def convert (camel_input):
    words = re.findall(r'[A-Z]?[a-z]+|[A-Z]{2,}(?=[A-Z][a-z]|\d|\W|$)|\d+', camel_input)
    return '_'.join(map(str.lower, words))


# Let's test it
test_strings = [
    'CamelCase',
    'camelCamelCase',
    'Camel2Camel2Case',
    'getHTTPResponseCode',
    'get200HTTPResponseCode',
    'getHTTP200ResponseCode',
    'HTTPResponseCode',
    'ResponseHTTP',
    'ResponseHTTP2',
    'Fun?!awesome',
    'Fun?!Awesome',
    '10CoolDudes',
    '20coolDudes'
]
for test_string in test_strings:
    print(convert(test_string))

Which outputs:

camel_case
camel_camel_case
camel_2_camel_2_case
get_http_response_code
get_200_http_response_code
get_http_200_response_code
http_response_code
response_http
response_http_2
fun_awesome
fun_awesome
10_cool_dudes
20_cool_dudes

The regular expression matches three patterns:

1. [A-Z]?[a-z]+: Consecutive lower-case letters that optionally start with an upper-case letter.
2. [A-Z]{2,}(?=[A-Z][a-z]|\d|\W|$): Two or more consecutive upper-case letters. It uses a lookahead to exclude the last upper-case letter if it is followed by a lower-case letter.
3. \d+: Consecutive numbers.

By using re.findall we get a list of individual "words" that can be converted to lower-case and joined with underscores.

Personally I am not sure how anything using regular expressions in python can be described as elegant. Most answers here are just doing "code golf" type RE tricks. Elegant coding is supposed to be easily understood.

def to_snake_case(not_snake_case):
    final = ''
    for i in xrange(len(not_snake_case)):
        item = not_snake_case[i]
        if i < len(not_snake_case) - 1:
            next_char_will_be_underscored = (
                not_snake_case[i+1] == "_" or
                not_snake_case[i+1] == " " or
                not_snake_case[i+1].isupper()
            )
        if (item == " " or item == "_") and next_char_will_be_underscored:
            continue
        elif (item == " " or item == "_"):
            final += "_"
        elif item.isupper():
            final += "_"+item.lower()
        else:
            final += item
    if final[0] == "_":
        final = final[1:]
    return final

>>> to_snake_case("RegularExpressionsAreFunky")
'regular_expressions_are_funky'

>>> to_snake_case("RegularExpressionsAre Funky")
'regular_expressions_are_funky'

>>> to_snake_case("RegularExpressionsAre_Funky")
'regular_expressions_are_funky'

''.join('_'+c.lower() if c.isupper() else c for c in "DeathToCamelCase").strip('_')
re.sub("(.)([A-Z])", r'\1_\2', 'DeathToCamelCase').lower()

Here's my solution:

def un_camel(text):
    """ Converts a CamelCase name into an under_score name. 

        >>> un_camel('CamelCase')
        'camel_case'
        >>> un_camel('getHTTPResponseCode')
        'get_http_response_code'
    """
    result = []
    pos = 0
    while pos < len(text):
        if text[pos].isupper():
            if pos-1 > 0 and text[pos-1].islower() or pos-1 > 0 and \
            pos+1 < len(text) and text[pos+1].islower():
                result.append("_%s" % text[pos].lower())
            else:
                result.append(text[pos].lower())
        else:
            result.append(text[pos])
        pos += 1
    return "".join(result)

It supports those corner cases discussed in the comments. For instance, it'll convert getHTTPResponseCode to get_http_response_code like it should.

For the fun of it:

>>> def un_camel(input):
...     output = [input[0].lower()]
...     for c in input[1:]:
...             if c in ('ABCDEFGHIJKLMNOPQRSTUVWXYZ'):
...                     output.append('_')
...                     output.append(c.lower())
...             else:
...                     output.append(c)
...     return str.join('', output)
...
>>> un_camel("camel_case")
'camel_case'
>>> un_camel("CamelCase")
'camel_case'

Or, more for the fun of it:

>>> un_camel = lambda i: i[0].lower() + str.join('', ("_" + c.lower() if c in "ABCDEFGHIJKLMNOPQRSTUVWXYZ" else c for c in i[1:]))
>>> un_camel("camel_case")
'camel_case'
>>> un_camel("CamelCase")
'camel_case'

So many complicated methods... Just find all "Titled" group and join its lower cased variant with underscore.

>>> import re
>>> def camel_to_snake(string):
...     groups = re.findall('([A-z0-9][a-z]*)', string)
...     return '_'.join([i.lower() for i in groups])
...
>>> camel_to_snake('ABCPingPongByTheWay2KWhereIsOurBorderlands3???')
'a_b_c_ping_pong_by_the_way_2_k_where_is_our_borderlands_3'

If you don't want make numbers like first character of group or separate group - you can use ([A-z][a-z0-9]*) mask.

A horrendous example using regular expressions (you could easily clean this up :) ):

def f(s):
    return s.group(1).lower() + "_" + s.group(2).lower()

p = re.compile("([A-Z]+[a-z]+)([A-Z]?)")
print p.sub(f, "CamelCase")
print p.sub(f, "getHTTPResponseCode")

Works for getHTTPResponseCode though!

Alternatively, using lambda:

p = re.compile("([A-Z]+[a-z]+)([A-Z]?)")
print p.sub(lambda x: x.group(1).lower() + "_" + x.group(2).lower(), "CamelCase")
print p.sub(lambda x: x.group(1).lower() + "_" + x.group(2).lower(), "getHTTPResponseCode")

EDIT: It should also be pretty easy to see that there's room for improvement for cases like "Test", because the underscore is unconditionally inserted.

Not in the standard library, but I found this module that appears to contain the functionality you need.