Python Cookbook

Adding an Entry to a Dictionary

Credit: Alex Martelli

Problem

Working with a dictionary D, you need to use the entry D[k] if it’s already present, or add a new D[k] if k isn’t yet a key in D.

Solution

This is what the setdefault method of dictionary objects is for. Say we’re building a word-to-page numbers index. A key piece of code might be:

theIndex = {}
def addword(word, pagenumber):
    if theIndex.has_key(word):
        theIndex[word].append(pagenumber)
    else:
        theIndex[word] = [pagenumber]

Good Pythonic instincts suggest substituting this “look before you leap” pattern with an “easier to get permission” pattern (see Recipe 5.4 for a detailed discussion of these phrases):

def addword(word, pagenumber):
    try: theIndex[word].append(pagenumber)
    except KeyError: theIndex[word] = [pagenumber]

This is just a minor simplification, but it satisfies the pattern of “use the entry if it is already present; otherwise, add a new entry.” Here’s how using setdefault simplifies this further:

def addword(word, pagenumber):
    theIndex.setdefault(word, []).append(pagenumber)

Discussion

The setdefault method of a dictionary is a handy shortcut for this task that is especially useful when the new entry you want to add is mutable. Basically, dict.setdefault(k, v) is much like dict.get(k, v), except that if k is not a key in the dictionary, the setdefault method assigns dict[k]=v as a side effect, in addition to returning v. (get would just return v, without affecting dict in any way.) Therefore, setdefault is appropriate ...