How to replace uppercase with underscore?

Question

I'm new to Python and I am trying to replace all uppercase-letters within a word to underscores, for example:

ThisIsAGoodExample

should become

this_is_a_good_example

Any ideas/tips/links/tutorials on how to achieve this?

Answer 1

Here's a regex way:

import re
example = "ThisIsAGoodExample"
print re.sub( '(?<!^)(?=[A-Z])', '_', example ).lower()

This is saying, "Find points in the string that aren't preceeded by a start of line, and are followed by an uppercase character, and substitute an underscore. Then we lower()case the whole thing.

Answer 2

import re
"_".join(l.lower() for l in re.findall('[A-Z][^A-Z]*', 'ThisIsAGoodExample'))

EDIT: Actually, this only works, if the first letter is uppercase. Otherwise this (taken from here) does the right thing:

def convert(name):
    s1 = re.sub('(.)([A-Z][a-z]+)', r'\1_\2', name)
    return re.sub('([a-z0-9])([A-Z])', r'\1_\2', s1).lower()

Answer 3

This generates a list of items, where each item is "_" followed by the lowercased letter if the character was originally an uppercase letter, or the character itself if it wasn't. Then it joins them together into a string and removes any leading underscores that might have been added by the process:

print ''.join('_' + char.lower() if char.isupper() else char
              for char in inputstring).lstrip('_')

BTW, you haven't specified what to do with underscores that are already present in the string. I wasn't sure how to handle that case so I punted.

Answer 4

example = 'ThisIsAGoodExample'
# Don't put an underscore before first character.
new_example = example[0].lower()
for character in example[1:]:
    # Append an underscore if the character is uppercase.
    if character.isupper():
        new_example += '_'
    new_example += character.lower()

Answer 5

As no-one else has offered a solution using a generator, here's one:

>>> sample = "ThisIsAGoodExample"
>>> def upperSplit(data):
...   buff = ''
...   for item in data:
...     if item.isupper():
...       if buff:
...         yield buff
...         buff = ''
...     buff += item
...   yield buff
...
>>> list(upperSplit(sample))
['This', 'Is', 'A', 'Good', 'Example']
>>> "_".join(upperSplit(sample)).lower()
'this_is_a_good_example'

Answer 6

An attempt at a readable version:

import re

_uppercase_part = re.compile('[A-Z][^A-Z]*')    

def uppercase_to_underscore(name):
    result = ''
    for match in _uppercase_part.finditer(name):
        if match.span()[0] > 0:
            result += '_'
        result += match.group().lower()
    return result

Answer 7

Parse your string, each time you encounter an upper case letter, insert an _ before it and then switch the found character to lower case