When I printed a list, I got a syntax error when using this method:
print i for i in [1,2,3]
I knew this is ok when using this method:
for i in [1, 2, 3]:
print i
and I knew that
(i for i in [1, 2, 3])
is a generator object, but I just don't get it that why
print i for i in [1, 2, 3]
does't work. Can anyone give me a clue?
6 Answers
The list comprehension syntax x for x in ... requires brackets around it. That's a syntactic rule of Python, just like requiring indentation, or requiring a colon after if or whatever. i for i in [1, 2, 3] by itself is not valid syntax. You need either [i for i in [1, 2, 3]] (for a list comprehension) or (i for i in [1, 2, 3]) (for a generator comprehension).
In Python 2, print is a statement, not an expression or a function, so you can't use it directly in a comprehension. Use this trick:
def f(x): print x
[f(i) for i in [1,2,3]]
Note that (f(i)...) doesn't work because this just creates a generator which would call f() if you iterated over it. The list comprehension [] actually invokes f().
[EDIT] If you use Python > 2.6, you can achieve the same using
from __future__ import print_function
[print(i) for i in [1, 2, 3]]
Note the () around the argument to print.
The list comprehension syntax ([expression for loop]) is a shorthand loop syntax for producing a list.
You are not producing a list, you want to print items in a loop. Since you are not producing a python list, you have to use a regular loop.
Alternatively, since all you are doing is printing the items on separate lines, just add the newlines yourself:
print '\n'.join(i for i in [1, 2, 3])
This produces the same output as:
for i in [1, 2, 3]:
print i
If you use Python 3, or use from __future__ import print at the top of your module and so use the print() function, you can send all values to the function in one call, and tell print() to use newlines in between:
values = [1, 2, 3]
print(*values, sep="\n")
As an expression (in the grammar):
[i for i in [1, 2, 3]]is a list comprehension.(i for i in [1, 2, 3])is a generator expression.- But
i for i in [1, 2, 3]by itself is a syntax error, and that's just the way it is. There must be something surrounding it. Unless you have(or[around it, it's not a validexpression, because theforkeyword is not valid at that point.
Inside the print statement, it wants an expression.
(As a red herring, func(i for i in [1, 2, 3]) is permitted as an expression, being a function call with the first argument being a generator expression.)
Print is not a function, it's a statement, and you can't have them in expressions. Just use a regular loop as you don't want to produce a list, that a list comprehension does. In theory you can do (not that you should. at all):
from __future__ import print_function
[print(my_item) for my_item in [1,2,3,4]]
1
2
3
4
Out[26]:
[None, None, None, None]
This is invalid python syntax. The i for i in [1, 2, 3] is only valid in a list or generator comprehension, ie. surrounded by [] or () respectively.
You'll want to use:
print '\n'.join(str(i) for i in [1, 2, 3])
