python dict setdefault, confused

Read the documentation for dict.setdefault: it is like get but if the key wasn't present then it is also set:

>>> my_dict = {}
>>> my_dict.setdefault('some key', 'a value')
'a value'
>>> my_dict
{'some key': 'a value'}
>>> my_dict.get('some key2', 'a value2')
'a value2'
>>> my_dict
{'some key': 'a value'}

Modifying a little your example:

>>> def what(*words):
...     d = dict()
...     for word in words:
...             curr = d
...             for letter in word:
...                     curr = curr.setdefault(letter, {})
...             curr = curr.setdefault('.', '.')
...             print 'curr is now: %r while d is %r' % (curr, d)
... 
>>> what('foo')
curr is now: '.' while d is {'f': {'o': {'o': {'.': '.'}}}}

As you can see curr changes, because when calling setdefault it sometimes(in your example always) create a new dict and set it as value to curr, while d always refers to the original dict. As you can see it is modified after the loop, since it's value is {'f': {'o': {'o': {'.': '.'}}}} which is quite different from {}.

Probably your confusion is due to the fact that curr = curr.setdefault(letter, {}) always create a new and empty dict, which is then assigned to curr(and thus for every letter you add a nesting level to the original dict instead of overwriting the values).

See this:

>>> my_dict = {}
>>> curr = my_dict
>>> for letter in 'foo':
...     print 'my_dict is now %r. curr is now %r' % (my_dict, curr)
...     curr = curr.setdefault(letter, {})
... 
my_dict is now {}. curr is now {}
my_dict is now {'f': {}}. curr is now {}
my_dict is now {'f': {'o': {}}}. curr is now {}
>>> my_dict
{'f': {'o': {'o': {}}}}

As you can see for every level the my_dict has a new nesting level.

Maybe, but I'm just guessing, you wanted to obtain something like 'foo' -> {'f': {}, 'o': {}}, in which case you should do:

>>> my_dict = {}
>>> for letter in 'foo':
...     my_dict.setdefault(letter, {})
... 
>>> my_dict
{'o': {}, 'f': {}}

d = dict() --> initializes an empty dictionary and binds it to the name d; so you have a dictionary object ({}) referenced by name d

Inside the outer for loop
curr = d --> binds another name curr to the same object. So, names (d and curr refer to the same object)

Inside the inner for loop
During the first iteration letter = 'f'

curr = curr.setdefault(letter, {})

There are 2 things that are happening in the above statement,

A) curr.setdefault(letter, {}) --> As per documentation:

"If key is in the dictionary, return its value. If not, insert key with a value of default and return default. default defaults to None.".

Since, the letter 'f' is not in the initial dictionary object it mutates the initial object to {'f':{}} and returns the value {}, which is not the initial dictionary object, but a new one that was created because of the setdefault statement. At this time both curr and d refer to the initial dictionary object which has since mutated to {'f':{}}.

B) Reassignment of the name curr to the return value mentioned above. Now, the names curr and d refer to different objects. d refers to the object {'f':{}}, while curr refers to an empty dictionary object, which is actually the value of d['f']. This is why the nesting happens in the original dictionary object, as we go through the loop.