While loops

Usage in Python

• When do I use them?

While loops, like the ForLoop, are used for repeating sections of code - but unlike a for loop, the while loop will not run n times, but until a defined condition is met. As the for loop in Python is so powerful, while is rarely used, except in cases where a user's input is required, for example:

n = raw_input("Please enter 'hello':")
while n.strip() != 'hello':
    n = raw_input("Please enter 'hello':")

However, the problem with the above code is that it's wasteful. In fact, what you will see a lot of in Python is the following:

while True:
    n = raw_input("Please enter 'hello':")
    if n.strip() == 'hello':
        break

As you can see, this compacts the whole thing into a piece of code managed entirely by the while loop. Having True as a condition ensures that the code runs until it's broken by n.strip() equaling 'hello'.

• Another version you may see of this type of loop uses while 1 instead of while True. This was due to past performance tweaks that are no longer relevant in current Python versions, and True is preferred for readability.

  • Starting with Py2.3, the interpreter optimizes while 1 to just a single jump. Using 1 is minutely faster, due to the ability to reassign True to a different value - the interpreter needs to look up the value of the variable, as opposed to loading a constant. As a programmer, it is up to you which style to use - but always remember that readability is important, and that speed is also important.

  • Starting in Python 3, True, False, and None are reserved words, so using while 1 here no longer provides any performance benefit.

  • See also: http://stackoverflow.com/questions/3815359/while-1-vs-for-whiletrue-why-is-there-a-difference