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Questions tagged [generator-expression]

A syntactic construct in Python providing a concise way to create a generator object, with syntax similar to a list comprehension.

Quoting the Python generator expression reference:

A generator expression is a compact generator notation in parentheses:

generator_expression ::=  "(" expression comp_for ")"

A generator expression yields a new generator object. Its syntax is the same as for comprehensions, except that it is enclosed in parentheses instead of brackets or curly braces.

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Generator expressions vs. list comprehensions

When should you use generator expressions and when should you use list comprehensions in Python? # Generator expression (x*2 for x in range(256)) # List comprehension [x*2 for x in range(256)]
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How does this input work with the Python 'any' function?

In the python docs page for any, the equivalent code for the any() function is given as: def any(iterable): for element in iterable: if element: return True return False How does this function know what element I wanna…
pythoniku
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Generator as function argument

Can anyone explain why passing a generator as the only positional argument to a function seems to have special rules? If we have: def f(*args): print "Success!" print args This works, as expected. >>> f(1, *[2]) Success! (1, 2) This does…
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yield in list comprehensions and generator expressions

The following behaviour seems rather counterintuitive to me (Python 3.4): >>> [(yield i) for i in range(3)] at 0x0245C148> >>> list([(yield i) for i in range(3)]) [0, 1, 2] >>> list((yield i) for i in range(3)) [0, None,…
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Unexpected behaviour with a conditional generator expression

I was running a piece of code that unexpectedly gave a logic error at one part of the program. When investigating the section, I created a test file to test the set of statements being run and found out an unusual bug that seems very odd. I tested…
Suraj Kothari
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Are list comprehensions syntactic sugar for `list(generator expression)` in Python 3?

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 =…
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List comprehension vs generator expression's weird timeit results?

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…
Ashwini Chaudhary
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next() doesn't play nice with any/all in python

I ran down a bug today that came about because I was using next() to extract a value, and 'not found' emits a StopIteration. Normally that would halt the program, but the function using next was being called inside an all() iteration, so the all…
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Python generator expression parentheses oddity

I want to determine if a list contains a certain string, so I use a generator expression, like so: g = (s for s in myList if s == myString) any(g) Of course I want to inline this, so I do: any((s for s in myList if s == myString)) Then I think it…
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Using while in list comprehension or generator expressions

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.…
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Unexpected output from list(generator)

I have a list and a lambda function defined as In [1]: i = lambda x: a[x] In [2]: alist = [(1, 2), (3, 4)] Then I try two different methods to calculate a simple sum First method. In [3]: [i(0) + i(1) for a in alist] Out[3]: [3, 7] Second…
Himanshu Mishra
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Differences between generator comprehension expressions

There are, as far as I know, three ways to create a generator through a comprehension1. The classical one: def f1(): g = (i for i in range(10)) The yield variant: def f2(): g = [(yield i) for i in range(10)] The yield from variant (that…
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Generator expressions Python

I have a list of dictionaries like the following: lst = [{'a': 5}, {'b': 6}, {'c': 7}, {'d': 8}] I wrote a generator expression like: next((itm for itm in lst if itm['a']==5)) Now the strange part is that though this works for the key value pair…
Apurva Kunkulol
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Use case for nested/multiple list comprehensions or generator expressions. When is it more elegant?

I see this kind of thing sometimes: (k for k in (j for j in (i for i in xrange(10)))) Now this really bends my brain, and I would rather it wasn't presented in this way. Are there any use-cases, or examples of having used these nested expressions…
Ali Afshar
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convert string to dict using list comprehension

I have came across this problem a few times and can't seem to figure out a simple solution. Say I have a string string = "a=0 b=1 c=3" I want to convert that into a dictionary with a, b and c being the key and 0, 1, and 3 being their respective…
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