I am experiencing a bit of confusion with how to place a for loop at the end of a line in python, for instance
for i in x:
print i
produces the expected result but if I run
print i for i in x
I get a syntax error. Could someone explain a little more about how one goes about putting your for loops at the end of a line like this.
In earlier versions of Python, the idea of list comprehensions was introduced to neaten up these two patterns of code:
# Go through each item in a list and run a test on them
# build a new list containing only items that pass the test
results = []
for item in somelist:
if sometest(item):
results.add(item)
and
# build a new list by changing every item in a list in the
# same way
results = []
for item in somelist:
results.add(2 * item)
by adding a new syntax that includes doing all three things in one - changing the items and/or testing them to only include some in the result, and creating the list of the results:
results = [2 * item for item in somelist if sometest(item)]
# results is a list
This feature uses the [] syntax that indicates "list" in Python, and it builds a list right away in memory.
Sometimes you don't want or need the entire list built in memory right away, so later versions of Python introduced generator expressions - the same idea, but they save memory by quickly returning a generator which you can iterate over as if it was a list, and it builds the list as and when you use it.
But they can't have the same syntax and be a direct swap out because they behave slightly differently, so they use () instead of [], e.g.:
somelist = [1,2,3,4,5]
results = (2 * item for item in somelist if sometest(item))
# results is a generator
If you try and call a function with this, you get two layers of parentheses:
function((2 * item for item in somelist))
Which looks silly, so you can leave one out:
function(2 * item for item in somelist)
Which appears to be a standalone backwards for loop on its own, but it actually isn't.
So with parentheses you can write this:
>>> print (item for item in [1,2,3])
<generator object <genexpr> at 0x7fe31b8663c0>
Which is the closest thing to what you wrote that is valid syntax in Python 2.x, but it doesn't do what you expect, and ^^ is what it does and why.
Or this:
>>> print [item for item in [1,2,3]]
[1,2,3]
Which generates a list and prints the list, the other close thing to what you wrote, but still not what you expected.
-- (there really isn't much point in me posting now a bunch other answers have appeared while I was writing this, but eh).
expression for i in x
doesn't mean anything by itself in Python. You can have
(expression for i in x)
[expression for i in x]
But you can't do
[print i for i in x]
because print isn't an expression, it's a statement.
First of all:
print is a function by default, or when you do from __future__ import print_function in Python 2.xHowever, if you insist on putting the for ... part after the print i part, you can do:
[print(i) for i in x]
(but I'm writing that example for purely "academic" purposes)
P.S. if you let us know what you want to do, we might be able to provide a suitable overall solution, but if you're just asking for the sake of it, then that's it.
You can't. You've probably seen a generator expression, which takes the form x for x in iter. a for loop is slightly different, though you can definitely see for x in iter inside the genexp.
In Python3, you can do:
print(*(i for i in x))
And as @wim points out in the comments, you can make it more "for loopy" by doing
print(*(i for i in x), sep='\n')
You can of course do arbitrary changes since this is a genexp, so e.g. i**2 for i in x will give you the square of each item in x, i for i in x if i%2 will give all odd numbers in x, etc.
This will create a generator of each item in x, then pass each one in turn (using the * assignment the same way *args is built and etc) as separate arguments to print
print(*x)?print(*(i for i in x if condition)) or etc.sep='\n' to make it more equivalent to the for loop, but +1 anyway.The only thing I can think of that is equivalent in python 2.x would be this:
print '\n'.join(str(i) for i in x)
But don't do that. The for loop as you had it is much clearer.
If you are just fooling around in interactive interpreter and want to one-liner it for some reason, this will work:
for i in x: print i
But it's violating pep8 so I wouldn't write a line like that into a script.
for i in x: print iis the clearest and most explicit way to do that.print(i for i in x), sinceprintis now a function.<generator object <genexpr> at 0x7f2d794b5500>print(list(i for i in x)).list(x)or[i for i in x]which is something else.