I'm having troubles understanding this. I have a dictionary, where the key is a tuple consisting of two strings. I want to lowercase the first string in this tuple, is this possible?
Since a tuple is immutable, you need to remove the old one and create a new one. This works in Python 2 and 3, and it keeps the original dict:
>>> d = {('Foo', 0): 0, ('Bar', 0): 0}
>>> for (k, j), v in list(d.items()): # list key-value pairs for safe iteration
... del d[k,j] # remove the old key (and value)
... d[(k.lower(), j)] = v # set the new key-value pair
...
>>> d
{('foo', 0): 0, ('bar', 0): 0}
Note, in Python 2, dict.items() returns a list copy, so passing it to list is unnecessary there, but I chose to leave it for full compatibility with Python 3.
You can also use a generator statement fed to dict, and let the old dict get garbage collected. This is also compatible with Python 2.7, 2.6, and 3.
>>> d = {('Foo', 0): 0, ('Bar', 0): 0}
>>> d = dict(((k.lower(), j), v) for (k, j), v in d.items())
>>> d
{('bar', 0): 0, ('foo', 0): 0}