Python - iterating over result of list.append

Question

why can't I do something like this:

files = [file for file in ['default.txt'].append(sys.argv[1:]) if os.path.exists(file)]

I presume you've imported os? But note: this is a list comprehension... not a generator expression. — johnsyweb
– johnsyweb, Commented Mar 22, 2011 at 11:30
What makes you think append() returns a value? Where did you read there? Where did you see any example like that? — S.Lott
– S.Lott, Commented Mar 22, 2011 at 14:37

Eli Bendersky · Accepted Answer · 2011-03-22 11:22:18Z

10

list.append doesn't return anything in Python:

>>> l = [1, 2, 3]
>>> k = l.append(5)
>>> k
>>> k is None
True

You may want this instead:

>>> k = [1, 2, 3] + [5]
>>> k
[1, 2, 3, 5]
>>>

Or, in your code:

files = [file for file in ['default.txt'] + sys.argv[1:] if os.path.exists(file)]

answered Mar 22, 2011 at 11:22

Eli Bendersky

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2 Comments

Martin Drlík Over a year ago

It is a fact to which I totally forgot

Eli Bendersky Over a year ago

@Martin: no worries, we all forget things. Which is why it's always best to write short code samples in the interactive prompt before writing long comprehensions

D.Shawley · Accepted Answer · 2011-03-22 11:29:39Z

4

You could also use itertools.chain if you don't want to duplicate lists.

files = [file for file in itertools.chain(['default.txt'], sys.argv[1:])
                  if os.path.exists(file)]

answered Mar 22, 2011 at 11:29

D.Shawley

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