Python course: Dictionaries

This is a way of creating mapping relationships in Python. Each element is composed of two parts: a key and a value. The syntax of a dict in Python is: {<key_1>: <value_1>, <key_2> : <value_2>, <key_n> : <value_n>}. See an example:

employee = { "name": "michael scott", "position": "regional manager", "age": 46, "past lovers": ["jan", "holly", "donna"], "married": False } print(employee["past lovers"]) # => ["jan", "holly", "donna"]  print(employee["past lovers"][0]) # => jan  print(employee.get("name")) # => michael scott  print(employee.get("branch", "scranton")) # => scranton  employee["gender"] = "male" # I can add new items  employee.update({"house owner": False}) # A new item can be added this way too  print(employee.get("gender")) # => male  print(employee.keys()) #=> dict_keys(['name', 'position', 'age', 'past lovers', 'married', 'gender', 'house owner'])  print(employee.values()) #=> dict_values(['michael scott', 'regional manager', 46, ['jan', 'holly', 'donna'], False, 'male', False])  

employee["past lovers"]

Note that this syntax is similar to the one used for getting elements by indexes in lists and tuples. But here, instead of an index, you use the "past lovers" key.

employee["past lovers"][0]

As you noticed, the value of employee["past lovers"] is a list containing three elements, so we can use the index to access a particular one.

employee.get("name")

This is an alternative way of accessing a value. It's the same as employee["name"].

employee.get("branch", "scranton")

Do you see a key named "branch" in employee? No. This syntax is a way of setting a default value in case a given key is missing. So if a "branch" key was present and it had a value "x", then "x" would be returned instead of "scranton". This is a handy feature in a variety of situations.

employee["gender"]

Dictionaries are mutable, which means you can add this new key "gender" to it. You can change them too.

employee.update({"house owner": False})

This is an alternative way of adding new items.

Dict comprehension

This concept assumes you have read the Loops, Deconstructing and Conditionals posts.

This is an interesting feature in Python that allows you to create dicts out of collections, such as lists or tuples. By using a more concise and readable syntax:

employees_list = [("angela", 50), ("michael", 46), ("pam", 30)] employees_dict = {k: v for k, v in employees_list} employees_dict_over_30 = {k: v for k, v in employees_list if v > 30} print(employees_dict) # => {'angela': 50, 'michael': 46, 'pam': 30} print(employees_dict_over_30) # => {'angela': 50, 'michael': 46} 

employees_dict

This is a dict created with dict comprehension. Let's go over its details in two parts:

1. Defines what will be in the new dict: k: v (k for the key and v for the value)
2. Defines the iteration over the original list: for k, v in employees_list (a tuple used as a iterator)

employees_dict_over_30

Also a dict created with dict comprehension. There's a third part now:

1. Defines what will be in the new dict: k: v (same as the previous example)
2. Defines the iteration over the original list: for k, v in employees_list (same as the previous example)
3. Defines a conditional for k: v to be added to the dict: if v > 30 (if the value of "v" is greater than 30)

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