Run length encoding in Python

I see many great solutions here but none that feels very pythonic to my eyes. So I'm contributing with a implementation I wrote myself today for this problem.

def run_length_encode(data: str) -> Iterator[Tuple[str, int]]:
    """Returns run length encoded Tuples for string"""
    # A memory efficient (lazy) and pythonic solution using generators
    return ((x, sum(1 for _ in y)) for x, y in groupby(data))

This will return a generator of Tuples with the character and number of instances, but can easily be modified to return a string as well. A benefit of doing it this way is that it's all lazy evaluated and won't consume more memory or cpu than needed if you don't need to exhaust the entire search space.

If you still want string encoding the code can quite easily be modified for that use case like this:

def run_length_encode(data: str) -> str:
    """Returns run length encoded string for data"""
    # A memory efficient (lazy) and pythonic solution using generators
    return "".join(f"{x}{sum(1 for _ in y)}" for x, y in groupby(data))

This is a more generic run length encoding for all lengths, and not just for those of over 4 characters. But this could also quite easily be adapted with a conditional for the string if wanted.

Rosetta Code has a lot of implementations, that should easily be adaptable to your usecase.

Here is Python code with regular expressions:

from re import sub

def encode(text):
    '''
    Doctest:
        >>> encode('WWWWWWWWWWWWBWWWWWWWWWWWWBBBWWWWWWWWWWWWWWWWWWWWWWWWBWWWWWWWWWWWWWW')
        '12W1B12W3B24W1B14W'    
    '''
    return sub(r'(.)\1*', lambda m: str(len(m.group(0))) + m.group(1),
               text)

def decode(text):
    '''
    Doctest:
        >>> decode('12W1B12W3B24W1B14W')
        'WWWWWWWWWWWWBWWWWWWWWWWWWBBBWWWWWWWWWWWWWWWWWWWWWWWWBWWWWWWWWWWWWWW'
    '''
    return sub(r'(\d+)(\D)', lambda m: m.group(2) * int(m.group(1)),
               text)

textin = "WWWWWWWWWWWWBWWWWWWWWWWWWBBBWWWWWWWWWWWWWWWWWWWWWWWWBWWWWWWWWWWWWWW"
assert decode(encode(textin)) == textin

Aside for setting a=i after encoding a sequence and setting a width for your int when printed into the string. You could also do the following which takes advantage of pythons groupby. Its also a good idea to use format when constructing strings.

from itertools import groupby

def runLengthEncode (plainText):
    res = []

    for k,i in groupby(plainText):
        run = list(i)
        if(len(run) > 4):
            res.append("/{:02}{}".format(len(run), k))
        else:
            res.extend(run)

    return "".join(res)

Just observe the behaviour:

>>> runLengthEncode("abcd")
'abc'

Last character is ignored. You have to append what you've collected.

>>> runLengthEncode("abbbbbcd")
'a/5b/5b'

Oops, problem after encoding. You should set a=i even if you found a long enough sequence.

Run length encoding in Python

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