for/if loops and a scope of the variable in python

Question

I am confused about the scope of the variable in python. Here is a toy example of what I am trying to do:

a = True
enumerated_set = enumerate(['tic','tac','toe'])
for i,j in enumerated_set:
    if a == True:
        print j

The result I get is:

tic
tac
toe

now,

print a

returns

`True`

and if I ran again

for i,j in enumerated_set:
    if a == True:
        print j

I get no output.

I am confused... Since globally a = True, why during the second loop the print was not executed.

I appreciate your help.

Edit: another example where I am confused

y = 'I like this weather'.split()
for item in y:
    for i,j in enumerated_set:
         if a == True: 
             print j

also produces no output....

You would need to re-create your generator created by enumerate every time you want to use it. — Rohit Jain
– Rohit Jain, Commented Nov 8, 2012 at 23:52

jdi · Accepted Answer · 2012-11-08 23:49:41Z

7

It is actually not a problem with your boolean. That is always True.

enumerated_set is a generator. Once you cycle through it, it is exhausted. You would need to create a new one.

In [9]: enumerated_set = enumerate(['tic','tac','toe'])

In [10]: enumerated_set.next()
Out[10]: (0, 'tic')

In [11]: enumerated_set.next()
Out[11]: (1, 'tac')

In [12]: enumerated_set.next()
Out[12]: (2, 'toe')

In [13]: enumerated_set.next()
---------------------------------------------------------------------------
StopIteration                             Traceback (most recent call last)
/usr/local/<ipython-input-13-7b0a413e4250> in <module>()
----> 1 enumerated_set.next()

StopIteration:

answered Nov 8, 2012 at 23:49

jdi

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5 Comments

notrockstar Over a year ago

Ohhh, now it makes sense. I was just going of the python documentation and didn't find any mention about it being exhaustive. Thank you!

Don Question Over a year ago

@notorckstart: don't forget to accept the answer ;-) he answered faster then i could comment! ;-)

jdi Over a year ago

It would have to be exhausted, otherwise it would never stop when you loop over it :-) No problem! I think you were thinking about it like being a function call, such as when you loop over a list and it calls the builtin method for looping over it's indexes each time. This is more like a "saved" loop that only evaluates as you ask it each time.

notrockstar Over a year ago

@DonQuestion, apparently now you have to wait for 10 minutes from the posting time to accept the answer. jdi, Thank you for saving my sanity!

Ben Over a year ago

The normal usage for enumerate is to call it anew each time you want to loop with index, not to save it to a variable; since the result of enumerate can only be used once in most circumstances, there's no point in giving it a name.

spencer nelson · Accepted Answer · 2012-11-08 23:51:59Z

1

This is not due to scope, it is due to the nature of enumerate, which creates a generator, not a list. Generators are single-use: they pop off elements in sequence, they don't create a list which can be evaluated again. This saves memory.

If you wanted to iterate over enumerated_set twice, you might do this:

enumerated_set = list(enumerate(['tic','tac','toe']))

answered Nov 8, 2012 at 23:51

spencer nelson

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Comments

jkalivas · Accepted Answer · 2012-11-08 23:52:47Z

1

It is nothing to do with the a variable. You are using an enumerator object,and in the first loop it is gone to its end. You have to recreate it for the second loop.

answered Nov 8, 2012 at 23:52

jkalivas

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