[Tutor] How and where to use pass and continue
bill.mill at gmail.com
Mon Mar 28 04:43:45 CEST 2005
On Sun, 27 Mar 2005 20:37:02 -0500, Kevin <python.programming at gmail.com> wrote:
> I am having lot of trouble learning where and when to use pass and
> continue. The two books that I use don't explian these very good. Is
> there a website the explains these is great detail?
> I have also looked at the python tutorial as well.
I'll try to help you out - pass and continue are pretty simple
concepts. Consider the following code snippet which I will try to use
to explain both:
command = None
while command != '3':
command = raw_input("Press 1 to pass, 2 to continue, or 3 to exit ")
if command == '1':
elif command == '2':
print "end of loop reached"
The 'pass' statement simply means 'do nothing'. In the example above,
when the python interpreter encounters the pass statement, it simply
continues with its execution as it normally would. It is usually used
as the only statement in the body of an if statement to denote
explicitly that nothing is to be done. I will often use it as a
placeholder so that a program compiles correctly, like:
#TODO: implement do_something_else()
Without the pass statement, there are no statements in the second
block, and python will raise a SyntaxError.
In the first example above, Python sees the pass, exits the series of
'If...elif..." conditions, advances to the final statement of the
while loop, prints "end of loop reached", and resumes execution at the
top of the loop.
The continue statement means what it says - continue with the loop,
but resume execution at the top of the loop. In the case of a while
loop, the exit condition will be evaluated again, and execution
resumes from the top. In the case of a for loop, the item being
iterated over will move to its next element. Thus,
for i in (1,2):
print "we never get here"
Will print 1, hit the continue, update i to the value 2, print 2, hit
the continue, and exit because there are no more iterations for i.
In the first example I gave, after python reaches the continue,
'command' is again evaluated to see if its value is 3, then the loop
proceeds from the top down. If you run the example, you should be able
to figure out what's going on.
There are a couple more wrinkles - for example, continue only works on
the innermost loop in its execution context - but generally, they work
as you expect. The longer you work with python, the more you'll find
this to be the case, but I'm biased.
Hope this helps, and feel free to ask questions about what you don't understand.
bill.mill at gmail.com
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