python,optimization , How can I optimize filename generator with underscore?


How can I optimize filename generator with underscore?

Question:

Tag: python,optimization

I have this code

def filename_generator(name, path=''):
    names = []

    for i in range(len(name)):
        p = '_'.join([x for x in name[i:]]) + path

        names.append(name[:i] + p)
        names += filename_generator(name[:i], p)

    return sorted(list(set(names)))


for e in filename_generator('longlonglonglonglonglongname'):
    print(e)

How can you see, i need generate all combinations of filename with underscore _.

Example:

Input:

name

Output:

n_a_m_e
n_a_me
n_am_e
n_ame
na_m_e
na_me
nam_e
name

My code works well, but very slow. How i can optimize algorithm of generation?


Answer:

Note that there are 2n-1 different such names for a string of lengh n, as there can or can not be a _ between any two characters. For your long-long name that makes 134,217,728 variants!

Regardless of what algorithm you use, this will take a very long time. One way to work around this, depending on your actual use case, might be to use a generator function. This way, you do not need to calculate all the variants, if maybe you need just a few of them (maybe just enough to find one that does not yet exist?). For example, like this:

def filename_generator(name):
    if len(name) > 1:
        first, rest = name[0], name[1:]
        for rest2 in filename_generator(rest):
            yield first + rest2
            yield first + "_" + rest2
    else:
        yield name

Output:

>>> list(filename_generator("name"))
['name', 'n_ame', 'na_me', 'n_a_me', 'nam_e', 'n_am_e', 'na_m_e', 'n_a_m_e']

Or, building upon the idea by @jacdeh, using binary numbers to determine where to put a _.

def filename_generator(name):
    d = {"0": "", "1": "_"}
    for n in range(2**(len(name)-1)):
        binary = "{:0{}b}".format(n, len(name))
        yield''.join(d[s] + c for c, s in zip(name, binary))

Or similar, using bit shifting:

def filename_generator(name):
    for n in range(2**(len(name)-1)):
        yield ''.join(c + ("_" if n & 1<<i == 1<<i else "") for i, c in enumerate(name))

However, either both those implementation are pretty lousy, or it does not help much in the end: They are both about 10 times slower than the recursive function, according to timeit.

Here's some timing information, using IPython's %timeit magic method:

In [8]: %timeit filename_generator("longname")         # your original function
1000 loops, best of 3: 610 µs per loop
In [9]: %timeit list(filename_generator1("longname"))  # my recursive generator
10000 loops, best of 3: 22.5 µs per loop
In [10]: %timeit list(filename_generator2("longname")) # my binary generator
1000 loops, best of 3: 322 µs per loop
In [11]: %timeit partition_string("longname")          # Eric's binary function
1000 loops, best of 3: 200 µs per loop

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