Записки программиста, обо всем и ни о чем. Но, наверное, больше профессионального.

2013-01-22

List comprehensions

Генерация списков в Python.
Тема довольно заезженная, но почему бы не обзавестись еще одной шпаргалкой?

>>> animal_names = [animal.name for animal in list_of_animals]
>>> squares = [x*x for x in range(15)]
>>> div_by_seven = [x for x in range(100) if x%7 == 0]
>>> moods = ['sad' if person.is_alone() else 'happy' for person in people_at_party]
>>> ident_matrix = [[1 if col_index==row_index else 0 for row_index in range(4)] for col_index in range(4)]
>>> flattened = [num for row in ident_matrix for num in row]
>>> even_zeroes = [num for row in ident_matrix if ident_matrix.index(row)%2==0 for num in row if num==0]

You can see how the awkward alternations between our assignments and our predicates quickly make our list comprehension much harder to read and follow. This is where list comprehensions begin to approach the limit of their usefulness. As much as it’s nice to have everything on one line, list comprehensions are meant as a convenience tool; if they’re making your code more complex and unreadable, they should be replaced by a more traditional structure. If your code looks indecipherable and fits on one line but could easily be replaced by a few more lines that could be written by a coder with only minimal experience with python, you’re trying too hard to impress someone.

>>> even_zeroes = []
>>> for row in ident_matrix[::2]:
>>>   for val in row:
>>>     even_zeroes.append(val) if val == 0 else None


Там еще про словари, про генераторы немножко. Годная статья.

original post http://vasnake.blogspot.com/2013/01/list-comprehensions.html

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