Assignment Condition in Python While Loop

Use break:

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

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

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

import functoolsfor 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)