generator-expression

In Python 3, is a list comprehension simply syntactic sugar for a generator expression fed into the list function? e.g. is the following code: squares = [x**2 for x in range(1000)] actually converted in the background into the following? squares = list(x**2 for x in range(1000)) I know the output is identical, and Python 3 fixes the surprising side-effects to surrounding namespaces that list comprehensions had, but in terms of what the CPython interpreter does under the hood, is the former converted to the latter, or are there any difference in how the code gets executed? Background I found

I can use if and for in list comprehensions/generator expressions as list(i for i in range(100) if i*i < 30) I know this is not the most efficient but bear with me as the condition could be much more complicated and this is just an example. However, this still goes through hundred iterations and only yields a value in the first 6. Is there a way to tell the generator expression where to stop with something like this: list(i for i in range(100) while i*i < 30) However, while is not understood in generator expressions. So, my question is, how do I write a generator expression with a stopping

The following test fails: #!/usr/bin/env python def f(*args): """ >>> t = 1, -1 >>> f(*map(lambda i: lambda: i, t)) [1, -1] >>> f(*(lambda: i for i in t)) # -> [-1, -1] [1, -1] >>> f(*[lambda: i for i in t]) # -> [-1, -1] [1, -1] """ alist = [a() for a in args] print(alist) if __name__ == '__main__': import doctest; doctest.testmod() In other words: >>> t = 1, -1 >>> args = [] >>> for i in t: ... args.append(lambda: i) ... >>> map(lambda a: a(), args) [-1, -1] >>> args = [] >>> for i in t: ... args.append((lambda i: lambda: i)(i)) ... >>> map(lambda a: a(), args) [1, -1] >>> args = [] >>> for

The following behaviour seems rather counterintuitive to me (Python 3.4): >>> [(yield i) for i in range(3)] <generator object <listcomp> at 0x0245C148> >>> list([(yield i) for i in range(3)]) [0, 1, 2] >>> list((yield i) for i in range(3)) [0, None, 1, None, 2, None] The intermediate values of the last line are actually not always None , they are whatever we send into the generator, equivalent (I guess) to the following generator: def f(): for i in range(3): yield (yield i) It strikes me as funny that those three lines work at all. The Reference says that yield is only allowed in a function

I was answering this question , I preferred generator expression here and used this, which I thought would be faster as generator doesn't need to create the whole list first: >>> lis=[['a','b','c'],['d','e','f']] >>> 'd' in (y for x in lis for y in x) True And Levon used list comprehension in his solution , >>> lis = [['a','b','c'],['d','e','f']] >>> 'd' in [j for i in mylist for j in i] True But when I did the timeit results for these LC was faster than generator: ~$ python -m timeit -s "lis=[['a','b','c'],['d','e','f']]" "'d' in (y for x in lis for y in x)" 100000 loops, best of 3: 2.36 usec