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?

Answer 1

Use break:

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

Answer 2

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)

Answer 3

没有functools的版本：

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

Answer 4

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

Answer 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.