在Python中分割字串时，值解包的怪异行为

Question

Use case: I've got a long string that is divided by linebreaks, and each line has two element separated by a comma.

Ideally, this should work

[(x, y) for line in lines.split() for x, y in line.split(',')]

But it doesn't, and yields a ValueError the same as below. So I tried to decompose the problem to figure out what's going on here

lines = \
"""a,b
c,d
e,f
g,h"""

lines = [line for line in lines.split()]

print(lines)
print(len(lines))
print([len(line) for line in lines])
print(all(',' in line for line in lines))

[(x, y) for l in lines for x,y in l.split(',')]

Yields:

/usr/bin/python3m /home/alex/PycharmProjects/test.py
['a,b', 'c,d', 'e,f', 'g,h']
4
[3, 3, 3, 3]
True

Traceback (most recent call last):
File "/home/alex/PycharmProjects/test.py", line 74, in <module>
...
File "/home/alex/PycharmProjects/test.py", line 63, in <listcomp>
[(x, y) for l in sines for x,y in l.split(',')]
ValueError: need more than 1 value to unpack

Yet if I replace the list comprehension in the last line with a classic for loop:

for line in lines:
x, y = line.split(',')

It executes successfully:

['a,b', 'c,d', 'e,f', 'g,h']
4
True
[3, 3, 3, 3]
a b
c d
e f
g h

This is driving me absolutely insane. If I further decompose it, I find that list, set, and generator comprehensions shit themselves trying to do this:

[(x,y) for x, y in "a,b".split(",")]

Anyone have an idea about why this occurs?

Answer 1

This code:

for x, y in "a,b".split(",")

is looking for two-item iterables that are inside the iterable (list) returned by "a,b".split(",") .

However, all it finds is 'a' and 'b' :

>>> "a,b".split(",")
['a', 'b']
>>>

Since both of these are only one-item iterables (strings with one character), the code breaks.

---

Considering the above, watch what happens when an extra character is added to each side of the comma:

>>> "ax,by".split(",")
['ax', 'by']
>>> [(x,y) for x, y in "ax,by".split(",")]
[('a', 'x'), ('b', 'y')]
>>>

As you can see, the code now works.

This is because "ax,by".split(",") returns an iterable (list) that contains two-item iterables (strings with two characters). Furthermore, this is exactly what for x, y in is looking for.

---

However, you could also place the last part in a tuple:

>>> ("a,b".split(","),)
(['a', 'b'],)
>>> [(x,y) for x, y in ("a,b".split(","),)]
[('a', 'b')]
>>>

("a,b".split(","),) returns an iterable (tuple) that contains two-item iterables (a list with two strings). Once again, this is exactly what for x, y in is looking for, so the code works.

---

With all this in mind, the below should fix your problem:

[(x, y) for line in lines.split() for x, y in (line.split(','),)]

Answer 2

why not this?

[tuple(l.split(',')) for l in lines ]

l.split(',') produces only two items for each l , not an iterable of two items per each l

Answer 3

Why not just:

lines = """
a,b
c,d
e,f
g,h
"""

lines = [line for line in lines.split()]

print(lines)
print(len(lines))
print([len(line) for line in lines])
print(all(',' in line for line in lines))

[l.split(",") for l in lines]