Question

I am confused how python is iterating through this dictionary. From python’s documentation, the itervalues returns an iterator over the dictionary’s values.

dict = {"hello" : "wonderful", "today is" : "sunny", "more text" : "is always good"}

for x in dict.itervalues():
    x = x[2:]   

print dict

This prints out the original dictionary unchanged. Why is that? If I am saying the value at position x is “blabla”, why is it not getting set?

Answer 1

This has nothing to do with strings or lists. The devil is in how the for is unfolded.

Doing

for x in d.iteritems():
    # loop body

is more-or-less equivalent to doing

iter = d.itervalues()
while True:
    try:
        x = next(iter)
        # loop body
    except StopIteration:
        break

So with this in mind it’s not very hard to see that we are just reassigning x, which holds a result from a function call.

iter = d.itervalues()
while True:
    try:
        x = next(iter)

        x = 5 # There is nothing in this line about changing the values of d
    except StopIteration:
        break