Same variable name for different values in nested loops.

Question

This code is perfectly valid Python

x=[[1,2,3,4], [11,22,33,44]]
for e in x:
    for e in e:
        print e

Can someone please tell me why

Answer 1

I suppose you could roughly translate the inner loop in that above expression to:

for e in x:
    ee = iter(e)
    try:
        e = next(ee)
        while True
            print e
            e = next(ee)
    except StopIteration
        pass

Note that the key thing here is in the statement: for e in ..., ... is converted to an iterator via the iterator protocol. The object you actually iterate over is a separate object from the e you gave it initially. Since it's a separate object (stored separately from its name in the current scope to allow it to be iterated over) there is no problem with binding a new variable to that name in the current scope -- Maybe I should say that there is no problem other than it makes the code really hard to follow.

It's effectively the same reason you don't have a problem doing this:

A = [['foo']]  #Define A
b = A[0]       #Take information from A and rebind it to something else
c = A          #We can even take the entire reference and bind/alias it to a new name.
A = 'bar'      #Re-assign A -- Python doesn't care that A already existed.

---

Here are a couple more things to think about:

x = [1,2,3,4]
for a in x:
    print a
    next(a)   #Raises an error because lists aren't iterators!

Now a seldom used, (but sometimes necessary) idiom:

x = [1,2,3,4]
y = iter(x)   #create an iterator from the list x
for a in y:
    print a
    #This next line is OK.  
    #We also consume the next value in the loop since `iter(y)` returns `y`!
    #In fact, This is the easiest way to get a handle on the object you're
    #actually iterating over.
    next(y)   

finally:

x = [1,2,3,4]
y = iter(x)   #create an iterator from the list x
for a in y:
    print a
    #This effectively does nothing to your loop because you're rebinding
    #a local variable -- You're not actually changing the iterator you're
    #iterating over, just as `A = 'bar'` doesn't change the value of
    #the variable `c` in one of the previous examples.
    y = iter(range(10))