Assignment Condition in Python While Loop

Question

In C, one can do

while( (i=a) != b ) { }

but in Python, it appears, one cannot.

while (i = sys.stdin.read(1)) != "\n":

generates

    while (i = sys.stdin.read(1)) != "\n":
         ^
SyntaxError: invalid syntax

(the ^ should be on the =)

Is there a workaround?

Solution 1

Use break:

while True:
    i = sys.stdin.read(1)
    if i == "\n":
       break
    # etc...

Solution 2

Starting Python 3.8, and the introduction of assignment expressions (PEP 572) (:= operator), it's now possible to capture an expression value (here sys.stdin.read(1)) as a variable in order to use it within the body of while:

while (i := sys.stdin.read(1)) != '\n':
  do_smthg(i)

This:

• Assigns sys.stdin.read(1) to a variable i
• Compares i to \n
• If the condition is validated, enters the while body in which i can be used

Solution 3

You can accomplish this using the built-in function iter() using the two-argument call method:

import functools
for i in iter(fuctools.partial(sys.stdin.read, 1), '\n'):
    ...

Documentation for this:

iter(o[, sentinel])
...
If the second argument, sentinel, is given, then o must be a callable object. The iterator created in this case will call o with no arguments for each call to its next() method; if the value returned is equal to sentinel, StopIteration will be raised, otherwise the value will be returned.

One useful application of the second form of iter() is to read lines of a file until a certain line is reached. The following example reads a file until the readline() method returns an empty string:

with open('mydata.txt') as fp:
    for line in iter(fp.readline, ''):
        process_line(line)

Solution 4

A version without functools:

for i in iter(lambda: sys.stdin.read(1), '\n'):

Solution 5

Personally I like imm's and Marks answers using break, but you could also do:

a = None
def set_a(x):
    global a
    a = x
    return a

while set_a(sys.stdin.read(1)) != '\n':
    print('yo')

though I wouldn't recommend it.

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Comments

[vastlysuperiorman]

@FalconMomot This seems like a perfectly reasonable pattern to me. What does it matter where the loop is broken? Either the terminating condition will happen, or it won't. If i == "\n" doesn't happen inside the loop (causing a break), it wouldn't have happened in the while-loop's condition argument either.

[David Hempy]

Yes, and thank you! Best answer here.