Python元组而不是列表

Question

I read some tricks about Python and met the following code. I confused that why the code create tuple with two elements in the list insted of list. Why python returns tuple in list instead of list in list? Is it related with are the tuples immutable? Thanks in advance.

>>>import os, glob
>>>[(os.stat(f).st_size, os.path.realpath(f)) for f in glob.glob('*.sh')]
[(3074, '/home/usr1/a.sh'),
(3386, '/home/usr1/b.sh'),
(3070, '/home/usr1/c.sh')]

Answer 1

The code explicitly generates tuples:

(os.stat(f).st_size, os.path.realpath(f))

That's a 2-value tuple for each list element generated. The list comprehension could be re-written as:

result = []
for f in glob.glob('*.sh'):
    item = (os.stat(f).st_size, os.path.realpath(f))
    result.append(item)

Replace the parenthesis with square brackets if you wanted to generate nested lists:

[[os.stat(f).st_size, os.path.realpath(f)] for f in glob.glob('*.sh')]

Answer 2

You create a tuple in Python by separating your items with commas:

>>> 1,2,3
(1, 2, 3)
>>> x = 1,2,3
>>> x
(1, 2, 3)

When using a list comprehension like you are, as well as other situations, you have to give python hints about the fact that you are trying to create a tuple (and it's tends to help other devs too, which is always a good thing). So as Martijin pointed out - (os.stat(f).st_size, os.path.realpath(f)) is creating a tuple.

>>>[(os.stat(f).st_size, os.path.realpath(f)) for f in glob.glob('*.sh')]
    ^                  ^.                   ^
    |                    \_ this makes      |
    \__________________     it a tuple      |
                       \____________________\___ These are how Python knows to
that the comma makes it a tuple - they group that piece of code toegether.

You could put anything in there you want...

• make it a list: [[os.stat(f).st_size, os.path.realpath(f)] for f in glob.glob('*.sh')]
• How about nothing? [None for f in glob.glob('*.sh')]
• Let's take the filename backwards: [f[::1] for f in glob.glob('*.sh')]

This is a pretty advanced resource that talks about things like list comprehensions and generators that may be an interesting read.

Answer 3

(a,b) is a tuple. [(a,b)] is a list of tuples.