Conditional for in Python

Question

Does Python have something like below?

for item in items #where item>3:
  #.....

I mean Python 2.7 and Python 3.3 both together.

georg · Accepted Answer · 2018-08-31T21:46:06.893

69

You can combine the loop with a generator expression:

for x in (y for y in items if y > 10):
    ....

itertools.ifilter (py2) / filter (py3) is another option:

items = [1,2,3,4,5,6,7,8]

odd = lambda x: x % 2 > 0

for x in filter(odd, items):
    print(x)

edited Aug 31 '18 at 21:46

answered Oct 20 '12 at 09:21

georg

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This is the right way to do it as no intermediate array is produced. This is called a generator expression. IMO its a pity you cant just write the shorter `for x in items if x>10:` though. – Michael Anderson Oct 20 '12 at 09:23
@MichaelAnderson: yeah, that would be neat. This syntax is said to have been rejected by Guido, I haven't found a proof though. – georg Oct 20 '12 at 09:41
@georg could you please elaborate in what would be the `itertools.ifilter` usage? – Sebastián Vansteenkiste Aug 31 '18 at 15:54
Oh! I didn't realize you meant the `filter` function, yes, that's more comftible for writing some things. I should probably start paying attention to performance, so far I've settled for making generator expressions as readable as possible, but I until I sit down and measure with clocks I probably won't know if my solutions are "good enough" – Sebastián Vansteenkiste Sep 03 '18 at 14:56

Rohit Jain · Answer 2 · 2012-10-20T09:24:04.920

12

You mean something like this: -

item_list = [item for item in items if item > 3]

Or, you can use Generator expression, that will not create a new list, rather returns a generator, which then returns the next element on each iteration using yield method: -

for item in (item for item in items if item > 3):
    # Do your task

edited Oct 20 '12 at 09:24

answered Oct 20 '12 at 09:18

Rohit Jain

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@J.F.Sebastian Ah sorry, should have `if` after for. – Rohit Jain Oct 20 '12 at 09:22

Mark Amery · Answer 3 · 2013-12-21T23:00:54.313

5

There isn't a special syntax like the where in your question, but you could always just use an if statement within your for loop, like you would in any other language:

for item in items:
    if item > 3:
        # Your logic here

or a guard clause (again, like any other language):

for item in items:
    if not (item > 3): continue

    # Your logic here

Both of these boring approaches are almost as succinct and readable as a special syntax for this would be.

edited Dec 21 '13 at 23:00

answered Oct 20 '12 at 09:38

Mark Amery

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The top-rated solution with a generator expression is more functional and indicates exactly what the list contains on the line of the for expression. – Neil Apr 17 '14 at 18:13

score 3 · Answer 4 · answered Jun 09 '17 at 21:16

Python 3 and Python 2.7 both have filter() function which allows extracting items out of a list for which a function (in the example below, that's lambda function) returns True:

>>> nums=[1,2,3,4,5,6,7,8]
>>> for item in filter(lambda x: x>5,nums):
...     print(item)
... 
6
7
8

Omitting function in filter() will extract only items that are True, as stated in pydoc filter

score 2 · Answer 5 · edited May 23 '17 at 10:30

You could use an explicit if statement:

for item in items:
    if item > 3:
       # ...

Or you could create a generator if you need a name to iterate later, example:

filtered_items = (n for n in items if n > 3)

Or you could pass it to a function:

total = sum(n for n in items if n > 3)

It might be matter of taste but I find a for-loop combined with inlined genexpr such as for x in (y for y in items if y > 3): to be ugly compared to the above options.

Conditional for in Python

5 Answers5