Question

In C++ often do something like this:

typedef map<int, vector<int> > MyIndexType; 

Where I then use it like this:

MyIndexType myIndex; for( ... some loop ...) {   myIndex[someId].push_back(someVal); } 

If there was no entry in the map the code will insert a new empty vector and then append to it.

In Python it would look like this:

myIndex = {}  for (someId,someVal) in collection:    try:       myIndex[someId].append(someVal)    except KeyError:       myIndex[someId] = [someVal] 

The try except is a bit ugly here. Is there a way to tell the dictionary an object type to insert when a KeyError is encountered at dictionary declaration time?

Answer 1

You want to use:

from collections import defaultdict myIndex = defaultdict(list) myIndex[someId].append(someVal) 

Standard Library defaultdict objects.

Example usage from the Python documentation:

>>> s = [('yellow', 1), ('blue', 2), ('yellow', 3), ('blue', 4), ('red', 1)] >>> d = defaultdict(list) >>> for k, v in s:         d[k].append(v)  >>> d.items() [('blue', [2, 4]), ('red', [1]), ('yellow', [1, 3])]