Assign variable in while loop condition in Python?

Question:

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.

Asked By: user541686

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Source

Answers:

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()
Answered By: dfb

You could do:

line = 1
while line:
    line = data.readline()
Answered By: brandon
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.

Answered By: shelhamer

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.

Answered By: Kirk Strauser

If you aren’t doing anything fancier with data, like reading more lines later on, there’s always:

for line in data:
    ... do stuff ...
Answered By: Ned Batchelder

If data has a function that returns an iterator instead of readline (say data.iterate), you could simply do:

for line in data.iterate():
    #...
Answered By: TorelTwiddler

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.

Answered By: rafalotufo

Try this one, works for files opened with open('filename')

for line in iter(data.readline, b''):
Answered By: Niklas Claesson

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.

Answered By: Mr. Deathless

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)
Answered By: Xavier Guihot

As of python 3.8 (which implements PEP-572) this code is now valid:

while line := data.readline():
   # do something with line
Answered By: Rich L