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?
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:
sys.stdin.read(1) to a variable ii to \nwhile body in which i can be usedUse break:
while True:
i = sys.stdin.read(1)
if i == "\n":
break
# etc...
i == "\n" doesn't happen inside the loop (causing a break), it wouldn't have happened in the while-loop's condition argument either.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 itsnext()method; if the value returned is equal to sentinel,StopIterationwill 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 thereadline()method returns an empty string:
with open('mydata.txt') as fp:
for line in iter(fp.readline, ''):
process_line(line)
A version without functools:
for i in iter(lambda: sys.stdin.read(1), '\n'):
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.
readline()compare toraw_input()?@nameto the comment text.while Trueinstead.