Differences between revisions 2 and 5 (spanning 3 versions)

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.

-  ⇤ ← Revision 2 as of 2010-12-22 00:11:12 → 
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  Editor: host86-154-237-157
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+   ← Revision 5 as of 2017-03-31 23:19:56 → ⇥
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  Editor: MarkZiesemer
  Comment: Account for Python 3 performance improvement.
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-''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 -
+''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:
-Line 9:
+Line 7:
-{{{#!python numbers=disable
n=raw_input('Please enter\'hello\':')
while n.strip()!='hello':
    n=raw_input('Please enter\'hello\':')
+{{{
n = raw_input("Please enter 'hello':")
while n.strip() != 'hello':
    n = raw_input("Please enter 'hello':")
-Line 15:
+Line 13:
-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 -
+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:
-Line 17:
+Line 15:
-{{{#!python numbers=disable
+{{{
-Line 19:
+Line 17:
-    n=raw_input('Please enter\'hello\':')
    if n.strip()=='hello':
+    n = raw_input("Please enter 'hello':")
    if n.strip() == 'hello':
-Line 23:
+Line 21:
-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() equalling 'hello'. Another version you may see of this type of loop uses ''1'' instead of ''True'' - this is exactly the same, though in Python 2.*, using while ''1'' is minutely faster, due to the ability to resign ''True'' to a different value.
+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.
   * [[https://docs.python.org/3.0/whatsnew/2.3.html#optimizations|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.
   * [[https://docs.python.org/3.0/whatsnew/3.0.html#changed-syntax|Starting in Python 3]], {{{True}}}, {{{False}}}, and {{{None}}} are reserved words, so using {{{while 1}}} here no longer provides any performance benefit.

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While loops

Usage in Python