How is the for-loop able to use a variable that isn't defined yet

Question

I'm new to coding and I'm a little confused. How/why can a for loop use a variable that isn't defined yet?

For example:

demond = {'green':'grass', 'red':'fire', 'yellow':'sun'}

for i in demond:
    print(i)

Output:

green
yellow
red

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score 0 · Answer 1 · answered Jul 17 '16 at 18:17

In python, you don't need to declare variables. In C/C++/JAVA etc. you will have to declare them first and then use them.

Variables are nothing but reserved memory locations to store values.Based on the data type of a variable, the interpreter allocates memory and decides what can be stored in the reserved memory.Python variables do not need explicit declaration to reserve memory space. The declaration happens automatically when you assign a value to a variable.

score 0 · Answer 2 · edited May 23 '17 at 12:32

There are two things that you need to keep in mind:

because Python is a weakly-typed language, you do not need to explicitly declare any variable to a certain object type. This is something you already know, and why you can assign things without having to state what type they will be.
For loop constructs do a lot of things in the background that you don't explicitly see. This means that although it doesnt LOOK like anything is being defined, it is.

With that in mind, I dont want to really explain how for loops work, because there are already answers available for that but the main point is that a for loop in python is the same as the following pseudo code.

#set up your iterable
demond = SOME_ITERABLE_OBJECT (this can be a list, string, dict, etc)

#this
for i in demond:
    do_something(i)

#is the same as this
i = demond[0] # the first item in demond
do_something(i)
i = demond[1] # the second item in demond
do_something(i)
i = demond[2] 
...
...
..
i = demond[n] # the last item in demond
do_something(i)

Now your follow up question may be this: what makes it so that, in your code, for i in demond sets i to equal to it's keys? Well that is just part of the design of python, specifically how dicts work. What the for loop is ACTUALLY doing is calling an iterables next() function until the iterable generator is done. Each iterable can have a different result from a for loop (see the first link).

NOTE: In my code example, I am setting i = demond[some_index]. This looks like a list index grab but it is really meant to just show that is iterating through the list in some sort of order. IT IS PSUEDO CODE. Just keep that in mind.

How is the for-loop able to use a variable that isn't defined yet

2 Answers2