28.02.18 00:27, Chris Angelico пише:
Example usage
=============
These list comprehensions are all approximately equivalent::
# Calling the function twice
stuff = [[f(x), f(x)] for x in range(5)]
The simplest equivalent of [f(x), f(x)] is [f(x)]*2. It would be worth
to use less trivial example, e.g. f(x) + x/f(x).
# Helper function
def pair(value): return [value, value]
stuff = [pair(f(x)) for x in range(5)]
# Inline helper function
stuff = [(lambda v: [v,v])(f(x)) for x in range(5)]
# Extra 'for' loop - see also Serhiy's optimization
stuff = [[y, y] for x in range(5) for y in [f(x)]]
# Expanding the comprehension into a loop
stuff = []
for x in range(5):
y = f(x)
stuff.append([y, y])
# Using a statement-local name
stuff = [[(f(x) as y), y] for x in range(5)]
Other options:
stuff = [[y, y] for y in (f(x) for x in range(5))]
g = (f(x) for x in range(5))
stuff = [[y, y] for y in g]
def g():
for x in range(5):
y = f(x)
yield [y, y]
stuff = list(g)
Seems the two last options are generally considered the most Pythonic.
map() and itertools can be helpful in particular cases.
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