CODEWITHSUNDEEP
pythonfor-loopnonetype
Oli
Oli

Reputation: 239790

For loop through the list unless empty?

I've been writing a lot of constructs like this the past couple of days:

things = get_list()
if things:
    for i in things:
        pass # do something with the list
else:
    pass # do something if the list was empty

Lot of junk and I assign the list to a real variable (keeping it in memory longer than needed). Python has simplified a lot of my code up til now... Is there a easy way to do this?

(My understanding is that the else in the for ... else: construct always triggers after it has looped, empty or not - so not what I want)

Upvotes: 43

Views: 80954

Answers (8)

user35288
user35288

Reputation:

Use a list comprehension:

def do_something(x):
  return x**2

things = []
result = [do_something(x) for x in things]
print result        # []

things = [1, 2, 3]
result = [do_something(x) for x in things]
print result       # [1, 4, 9]

Upvotes: 11

Aaron D. Marasco
Aaron D. Marasco

Reputation: 6748

I get that it's been about 15 years since it was asked, but now Python has the "walrus operator" to do exactly what you asked for with only a slight tweak to your original code:

if my_list := get_list():
    for i in my_list:
        pass  # do something with the element
else:
    pass  # do something if the list was empty

There's no strong scoping like in other languages tho, so my_list will still be around after the statement, but you've clearly expressed your intent.

Upvotes: 0

Laserwhit
Laserwhit

Reputation: 1

i = None
for i in get_list():
    pass # do something with the list
else:
    if i is None:
        pass # do something if the list was empty

Does that help? Yes I know we are two years away from the need :-)

Upvotes: 0

Jiri
Jiri

Reputation: 16625

I think your way is ok in general case, but you may consider this approach:

def do_something(item):
   pass # do something with the list

def action_when_empty():
   pass # do something if the list was empty

# and here goes your example
yourlist = get_list() or []
another_list = [do_something(x) for x in yourlist] or action_when_empty()

Upvotes: 1

Tom Leys
Tom Leys

Reputation: 19029

Based on the other answers, I think the cleanest solutions are

#Handles None return from get_list
for item in get_list() or []: 
    pass #do something

or the comprehension equiv

result = [item*item for item in get_list() or []]

Upvotes: 89

ilya n.
ilya n.

Reputation: 18816

If your actions are different, I would do:

list_ = get_list() # underscore to keep built-in list
if not list_:
    # do something
for i in list_: #
    # do something for each item

If your actions are similar, this is more beautiful:

for i in list_ or [None]:
   # do something for list item or None

or, if you might have None as a list element, 

for i in list_ or [...]:
   # do something for list item or built-in constant Ellipsis

Upvotes: 4

Kenan Banks
Kenan Banks

Reputation: 211924

Slighty more terse is:

for i in my_list:
    # got a list
if not my_list:
    # not a list

assuming you are not changing the length of the list in the loop.

Edit from Oli: To compensate my worries of memory use, it would want withing:

with get_list() as my_list:
    for i in my_list:
        # got a list
    if not my_list:
        # not a list

But yes, that's quite a simple way around the issue.

Upvotes: 5

Jonathan Feinberg
Jonathan Feinberg

Reputation: 45324

def do_something_with_maybe_list(maybe_list):
    if maybe_list:
        for x in list:
            do_something(x)
    else:
        do_something_else()

do_something_with_maybe_list(get_list())

You could even extract the actions to be done:

def do_something_with_maybe_list(maybe_list, process_item, none_action):
    if maybe_list:
        for x in list:
            process_item(x)
    else:
        none_action()

do_something_with_maybe_list(get_list(), do_something, do_something_else)
do_something_with_maybe_list(get_otherlist(), do_other, do_still_other)

Edit from Oli: Or go one further:

def do_something_with_maybe_list(maybe_list, process_item, none_action):
    if maybe_list:
        return process_list(maybe_list)
    return none_action()

do_something_with_maybe_list(get_list(), do_something, do_something_else)
do_something_with_maybe_list(get_otherlist(), do_other, do_still_other)

Upvotes: 2

Related Questions