How to get itertool.product to generate strings instead of list of chars and then brace each string with additional strings?

Question

I don’t know much about python data structures but basically I’m generating all permutations of letters :

import, itertools, string    
k = itertools.product(string.ascii_lowercase, repeat = 2)
list(k)
[('a', 'a'), ('a', 'b'), ('a', 'c'), ..., ('z', 'x'), ('z', 'y'), ('z', 'z')]

I need to perform 2 operations on the generator k that will preserve it as a generator if possible :

1) Concatenate each list so list(k) would return :

["aa", "ab", ..., "zy", "zz"]

2) Brace each string with aditional strings "str1" and "str2" so after step 1) and step 2) list(k) would return :

["str1aastr2", "str1abstr2", ...,"str1zystr2","str1zzstr2"]

How to proceed to get a generator that looks like that so I can feed it to scrapy’s start_urls ?

Asked By: ChiseledAbs

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Source

Answer 1

You can use a generator expression to do the complete action. Not sure I understand the need to keep it as a generator if you immediately call list() on it:

>>> import itertools as it
>>> from string import ascii_lowercase
>>> k = ('str1{}str2'.format(''.join(s)) for s in it.product(ascii_lowercase, repeat=2))
>>> next(k)
'str1aastr2'
>>> list(k)
['str1abstr2', 'str1acstr2', 'str1adstr2', 'str1aestr2', ...]

Note: the str1aastr2 was consumed by the next(k)

Or a slightly different construct:

>>> k = (f'str1{c1}{c2}str2' for c1, c2 in it.product(ascii_lowercase, repeat=2))
>>> next(k)
'str1aastr2'
>>> next(k)
'str1abstr2'

Answered By: AChampion

Answer 2

You can create a new generator that will produce the desired values:

import itertools
import string

def g():
    start = ["str1"]
    end = ["str2"]
    for item in itertools.product(string.ascii_lowercase, repeat=2):
        yield "".join(start + list(item) + end)

Example:

>>> gen = g()
>>> list(gen)[:10]
['str1aastr2', 'str1abstr2', 'str1acstr2', 'str1adstr2', 'str1aestr2', 'str1afstr2', 'str1agstr2', 'str1ahstr2', 'str1aistr2', 'str1ajstr2']

After gen = g() you’ve got a generator object that you can use with Scrapy.

Answered By: Simeon Visser

How to get itertool.product to generate strings instead of list of chars and then brace each string with additional strings?

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