Assign variable in while loop condition in Python?
PythonWhile LoopVariable AssignmentPython Problem Overview
I just came across this piece of code
while 1:
line = data.readline()
if not line:
break
#...
and thought, there must be a better way to do this, than using an infinite loop with break
.
So I tried:
while line = data.readline():
#...
and, obviously, got an error.
Is there any way to avoid using a break
in that situation?
Edit:
Ideally, you'd want to avoid saying readline
twice... IMHO, repeating is even worse than just a break
, especially if the statement is complex.
Python Solutions
Solution 1 - Python
Starting Python 3.8
, and the introduction of assignment expressions (PEP 572) (:=
operator), it's now possible to capture the condition value (data.readline()
) of the while loop as a variable (line
) in order to re-use it within the body of the loop:
while line := data.readline():
do_smthg(line)
Solution 2 - Python
Try this one, works for files opened with open('filename')
for line in iter(data.readline, b''):
Solution 3 - Python
If you aren't doing anything fancier with data, like reading more lines later on, there's always:
for line in data:
... do stuff ...
Solution 4 - Python
This isn't much better, but this is the way I usually do it. Python doesn't return the value upon variable assignment like other languages (e.g., Java).
line = data.readline()
while line:
# ... do stuff ...
line = data.readline()
Solution 5 - Python
Like,
for line in data:
# ...
? It large depends on the semantics of the data
object's readline semantics. If data
is a file
object, that'll work.
Solution 6 - Python
for line in data:
... process line somehow....
Will iterate over each line in the file
, rather than using a while
. It is a much more common idiom for the task of reading a file in my experience (in Python).
In fact, data
does not have to be a file but merely provide an iterator.
Solution 7 - Python
As of python 3.8 (which implements PEP-572) this code is now valid:
while line := data.readline():
# do something with line
Solution 8 - Python
If data
is a file, as stated in other answers, using for line in file
will work fine. If data is not a file, and a random data reading object, then you should implement it as an iterator, implementing __iter__
and next
methods.
The next
method should to the reading, check if there is more data, and if not, raise StopIteration
. If you do this, you can continue using the for line in data
idiom.
Solution 9 - Python
According to the FAQ from Python's documentation, iterating over the input with for
construct or running an infinite while True
loop and using break
statement to terminate it, are preferred and idiomatic ways of iteration.
Solution 10 - Python
You could do:
line = 1
while line:
line = data.readline()
Solution 11 - Python
If data
has a function that returns an iterator instead of readline
(say data.iterate
), you could simply do:
for line in data.iterate():
#...