Dictionary.get()

Like enumerate, I'm not sure that this can be rightly called a trick since it's just a built-in function, but it's one that I wasn't aware of until long after I first needed it. Using a dictionary's get() function allows you to automatically check whether a key is in a dictionary and return a default value if it isn't.

For example:

>>> numbers = {1: 'one', 2:'two', 3:'three'}
>>> print numbers.get(1, 'Number not defined')
one
>>> print numbers.get(4, 'Number not defined')
Number not defined

This is useful in all sorts of situations. One common situation in which I find it useful is when you want to get counts of the numbers of items in a group of items, for example, if you want to create a histogram. For example, to count the frequency of letters in a string:

my_string = "I want to get the counts for each letter in this sentence"
counts = {}

for letter in my_string:
    counts[letter] = counts.get(letter, 0) + 1
print counts

Here, for each letter in my_string, you are getting the number of counts for that letter; if that letter hasn't yet been added to the counts dictionary, then the 0 is returned.