Say I have a function:
x=[]
i=5
while i<=20:
x.append(i)
i=i+10
return x
Is there a way to convert it to a list comprehension like this?
newList = [i=05 while i<=20 i=i+10]
I get a syntax error.
You don't need a list comprehension for that. range will just do:
list(range(5, 21, 10)) # [5, 15]
A while loop is not possible inside of a list comprehension. Instead, you could do something like this:
def your_while_generator():
i = 5
while i <= 20:
yield i
i += 10
[i for i in your_while_generator()]
No, you cannot use while in a list comprehension.
From the grammar specification of Python, only the following atomic expressions are allowed:
atom: ('(' [yield_expr|testlist_comp] ')' | '[' [testlist_comp] ']' | '{' [dictorsetmaker] '}' | NAME | NUMBER | STRING+ | '...' | 'None' | 'True' | 'False')
The expression corresponding to a list comprehension - testlist_comp looks like the following in Python 3:
testlist_comp: (test|star_expr) ( comp_for | (',' (test|star_expr))* [','] )
Here, the only statements allowed are
test: or_test ['if' or_test 'else' test] | lambdef
star_expr: '*' expr
comp_for: [ASYNC] 'for' exprlist 'in' or_test [comp_iter]
where
comp_if: 'if' test_nocond [comp_iter]
comp_iter: comp_for | comp_if
There is not a single while statement allowed anywhere. The only keywords you are allowed to use is a for, for a for loop.
Use a for loop, or take advantage of itertools.
There isn't any syntax for this, but you can use itertools. For example:
In [11]: from itertools import accumulate, repeat, takewhile
In [12]: list(takewhile(lambda x: x <= 20, accumulate(repeat(1), lambda x, _: x + 10)))
Out[12]: [1, 11]
(That's not Pythonic though. The generator solution or explicit solution should be preferred.)