Working with dictionaries

Printing Dictionaries

Although dictionaries are a new data structure, we can still use a lot of Python knowledge we already have to get the most out of them. Dictionaries work just fine with print(), and we can use the print() function on dictionaries to easily check their contents. We can simply call print() on a dictionary the same way we do for int, string, or list datatypes. Here’s an example with the dictionary from the previous steps.

class_dict = {'instructors': 1, 'CAs': 2, 'students': 59}

If we run print(class_dict), we would get the output:

{'instructors': 1, 'CAs': 2, 'students': 59}

Note that dictionaries are enclosed by curly braces {} when being printed.

Iterating over Dictionaries with a For Loop

We can use for loops on dictionaries in a very similar way to lists. Using a for loop over a dictionary will iterate over the keys of the dictionary. Let’s use a for loop to print all the keys in class_dict.

for key in class_dict:
    print(key)

The above code would output:

instructors
CAs
students

Note: As of Python 3.5, the order of items in a dictionary will be consistent. Since we defined class dict in the order instructors, CAs, students, that is the order they will be iterated over in the for loop. However, in older versions of Python, the order will be arbitrary, so be careful. In Python 2.6 for example, the order could be any combination of instructors, CAs, students, and we wouldn’t know until we ran the loop.

Say we want to print all key : value pairs in a dictionary. How will we go about that? We can do a loop like this:

for key in class_dict:
    print(key + " : " + str(class_dict[key]))

Try it for yourself and observe the output.

Using the “in” Operator

Similar to how we can use the in operator to check for list membership, we can also use the same in operator to check if a particular key is in a dictionary. For example, the following line:

"CAs" in class_dict

evaluates to True.

Similarly,

"vuvuzelas" in class_dict

evaluates to False.

Note that the in operator on a dictionary will only check for keys, not values. So,

"vuvuzelas" in class_dict

evaluates to False.

Note that the in operator on a dictionary will only check for keys, not values. So,

2 in class_dict

will evaluate to False, even though 2 is a value in the dictionary associated with the key "CAs".