Question

Like below, why does not the final r print [{'k': 1}, {'k': 2}] ?

l = [1, 2]
d = {}
r = []

for el in l:
    d['k'] = el
    print '> ' + str(d)
    r.append(d)

print r  

OUTPUT:

> {'k': 1}
> {'k': 2}
[{'k': 2}, {'k': 2}]

It is printed as wished before append operation, however after the dict is appended to a list, why does it append the last element repeatedly?

Answer 1

You’re assigning a new value to the same key when you pass through the loop a second time.

for el in l:
    d['k'] = el
    print '> ' + str(d)
    r.append(d)

print r  

The first time through the loop, el is 1. d['k'] is set to 1, and then printed. Then d is appended to r.

The second time through the loop, el is 2. d['k'] — the very same value of the very same key of the very same dictionary — is set to 2. Then the very same dictionary, d, is appended to r.

Now r is the same as if you had said

r = [d, d]

And since d['k'] is now 2, both references to d in the list r yield {'k': 2}.

If you want it to be different, you have to create a new dictionary every time through the loop.