Dict

In Python a dictionary can be created with an expression in curly brackets. But the dict built-in method can also be used. This provides more initialization options.

With dict, we create an empty dictionary. We create a dictionary from another dictionary (copying it). We create one from a list.

Empty dictionary example

Here we create an empty dictionary with dict(). No keys or values are inside the dictionary. Its len is 0.

# Use dict to create empty dictionary.
values = dict()

# Add value and get value.
values["cat"] = 1
print(values.get("cat"))
1

Copy dictionary

Here we copy a dictionary with dict. We create a dictionary called values and then copy it into a dictionary called "copy."

values = {"cat": 1, "bird": 2}

# Copy the dictionary with the dict copy constructor.
copy = dict(values)

# Change copy.
copy["cat"] = 400

# The two dictionaries are separate.
print(values)
print(copy)
{'bird': 2, 'cat': 1}
{'bird': 2, 'cat': 400}

List, dict

A list of tuples (each a key-value pair) can be used to construct a dictionary. Dict turns these pairs into keys and values. This is a way to transform a list into a dictionary.

# Create a dictionary with dict based on a list of pairs.
# ... List contains tuples with keys and values.
values = [("cat", 1), ("bird", 200)]
lookup = dict(values)

print(values)
print(lookup.get("cat"))
print(lookup.get("bird"))
[('cat', 1), ('bird', 200)]
1
200

Named arguments

Python supports named arguments. In this syntax, arguments are passed with an equals sign statement. A dictionary can be constructed with this syntax.

# Create a dictionary with named arguments.
animals = dict(bird=1, dog=2, fish=9)

print(animals.get("bird"))
print(animals.get("dog"))
print(animals.get("fish"))
1
2
9

Dict TypeError

The dict keyword must not be incorrectly used. If we provide incorrect arguments, a TypeError will interrupt our program's execution.

# The dict built-in must be called with a valid argument.
result = dict(1)
Traceback (most recent call last):
  File "C:\programs\file.py", line 5, in <module>
    result = dict(1)
TypeError: 'int' object is not iterable

Dictionary copy

We can invoke the copy() method on a dictionary to perform a copy. This has the same result as using the dict built-in method.

With this function, we create a powerful dictionary collection from varied arguments. Often with dictionaries we do not need a dict() call. But when we do, dict simplifies our code.