Is there a "do ... until" in Python? [duplicate]

Is there a

do until x: ...

in Python, or a nice way to implement such a looping construct?

4

4 Answers

There is no do-while loop in Python.

This is a similar construct, taken from the link above.

 while True: do_something() if condition(): break

7

I prefer to use a looping variable, as it tends to read a bit nicer than just "while 1:", and no ugly-looking break statement:

finished = False
while not finished: ... do something... finished = evaluate_end_condition()

1

There's no prepackaged "do-while", but the general Python way to implement peculiar looping constructs is through generators and other iterators, e.g.:

import itertools
def dowhile(predicate): it = itertools.repeat(None) for _ in it: yield if not predicate(): break

so, for example:

i=7; j=3
for _ in dowhile(lambda: i<j): print i, j i+=1; j-=1

executes one leg, as desired, even though the predicate's already false at the start.

It's normally better to encapsulate more of the looping logic into your generator (or other iterator) -- for example, if you often have cases where one variable increases, one decreases, and you need a do/while loop comparing them, you could code:

def incandec(i, j, delta=1): while True: yield i, j if j <= i: break i+=delta; j-=delta

which you can use like:

for i, j in incandec(i=7, j=3): print i, j

It's up to you how much loop-related logic you want to put inside your generator (or other iterator) and how much you want to have outside of it (just like for any other use of a function, class, or other mechanism you can use to refactor code out of your main stream of execution), but, generally speaking, I like to see the generator used in a for loop that has little (ideally none) "loop control logic" (code related to updating state variables for the next loop leg and/or making tests about whether you should be looping again or not).

2

No there isn't. Instead use a while loop such as:

while 1: ...statements... if cond: break

11