When the key is a tuple in dictionary in Python
Question:
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?
Answers:
You would have to change the string to lowercase before putting it into the tuple. Tuples are read-only after they are created.
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}
You can use a dict comprehension to build a new dict with your changes applied:
d = { (a.lower(), b) : v for (a,b), v in d.items() }
Tuples are "immutable", which means you cannot change a tuple in place after it is created. However, you can make a new tuple by modifying the existing tuple. I used the word "immutable" because that is the Pythonic word, better than "read only".
Lists are mutable. Interestingly enough, strings are immutable, but that rarely causes any problem.
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?
You would have to change the string to lowercase before putting it into the tuple. Tuples are read-only after they are created.
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}
You can use a dict comprehension to build a new dict with your changes applied:
d = { (a.lower(), b) : v for (a,b), v in d.items() }
Tuples are "immutable", which means you cannot change a tuple in place after it is created. However, you can make a new tuple by modifying the existing tuple. I used the word "immutable" because that is the Pythonic word, better than "read only".
Lists are mutable. Interestingly enough, strings are immutable, but that rarely causes any problem.