Dictionaries

The built-in Python dictionary class dict is an unordered collection of elements, each of which has a unique key and a value. A key can be any immutable object, but a string is most common. A value can be any object. The basic syntax to create a dict object with keys k\(x\) and values v\(x\) is {k1: v1, k2: v2, ...}. For instance, we can define a dict as follows:

d = {"foo": 5, "bar": 1, "baz": -3}

Accessing a value requires its key. To access a value in dictionary d with key k, use the syntax d[k]. For example,

d = { # It is often useful to break lines at each key-value pair
    "name": "Spiff", 
    "age": 33, 
    "occupation": "spaceman",
    "enemies": ["Zorgs", "Zargs", "Zogs"]
}
print(f"{d['name']} is a {d['age']} year old"
      f"{d['occupation']} who fights {d['enemies'][0]}.")

This returns

Spiff is a 33 year old spaceman who fights Zorgs.

A value v with key k can be added to an existing dictionary d with the syntax d[k] = v. For instance, (Filik et al. 2019)

d = {} # Empty dictionary
d["irony"] = "The use of a word to mean its opposite."
d["sarcasm"] = "Irony intended to criticize."

Dictionaries are mutable; therefore, we can change their contents, as in the following example:

d = {}
d["age"] = 33   # d is {"age": 33}
d["age"] = 31   # d is {"age": 31}

Dictionaries have several handy methods; these are listed in tbl. 1.

Note that most of these methods apply to dictionary instance d, either mutating d or returning something from d. However, the fromkeys() method is called from the class dict because it has nothing to do with an instance. Such methods are called class methods; the other methods we’ve considered thus far are instance methods.

Dictionary view objects—returned by items(), keys(), and values()—are dynamically updating objects that change with their dictionary. For instance,

d = {"a": 1, "b": 2}
d_keys = d.keys()
print(f"View object before: {d_keys}")
d["c"] = 3
print(f"View object after: {d_keys}")

This returns

View object before: dict_keys(['a', 'b'])
View object after: dict_keys(['a', 'b', 'c'])

View objects can be converted to lists with the list() function, as in list(d_keys).

names = ["Mo", "Jo", "Flo"]
data = dict.fromkeys(names) # => {"Mo": None, "Jo": None, "Flo": None}

#%% Populate Data
data["Mo"] = {}
data["Mo"]["year"] = "sophomore"
data["Mo"]["major"] = "Mechanical Engineering"
data["Mo"]["GPA"] = 3.44
data["Jo"] = {}
data["Jo"]["year"] = "junior"
data["Jo"]["major"] = "Computer Science"
data["Jo"]["GPA"] = 3.96
data["Flo"] = {}
data["Flo"]["year"] = "sophomore"
data["Flo"]["major"] = "Philosophy"
data["Flo"]["GPA"] = 3.12

#%% Data Operations and Printing
print(f"Mo is a {data['Mo']['year']}. "
      f"Jo is a {data['Jo']['year']}. "
      f"Flo is a {data['Flo']['year']}.")
data["Jo"]["GPA"] = 3.98
print(f"Jo's new GPA is {data['Jo']['GPA']}")
data.pop("Mo")
print(f"Names sans Mo: {list(data.keys())}")

Mo is a sophomore. Jo is a junior. Flo is a sophomore.
Jo's new GPA is 3.98
Names sans Mo: ['Jo', 'Flo']