Assignement in Python with list.extend()

This is weird for me :

d = ['foo', 'bar', 'ba', 'c']
print d
f = d
f.extend(d)
print d

give me the result

python “/tmp/a.py”
[‘foo’, ‘bar’, ‘ba’, ‘c’]
[‘foo’, ‘bar’, ‘ba’, ‘c’, ‘foo’, ‘bar’, ‘ba’, ‘c’]

Compilation finished at Mon Jul 31 16:49:4

It seems that extend assign as well the non extended part (d) which is confusing because to merge list i need to use temporary variable.