1. Collection initialization

When initializing empty Python collections such as lists and dictionaries, favor using the corresponding literal delimiters instead of the corresponding functions.

Literals are shorter, more familiar to most readers, and emphasize the shape of the value immediately. They also avoid looking like a conversion from another iterable when you are simply creating an empty collection.

1.1. Don’t do this

names = list() # initialize list
data = dict()  # initialize dictionary

1.2. Do this

names = [] # initialize list
data = {}  # initialize dictionary

Using the literal delimiters [] and {} is faster and more Pythonic than using list() and dict(). The literal form is faster because the generated bytecode does not need to look up a function name.

You can inspect the bytecode for [] as follows.

from dis import dis

dis('[]')

The bytecode looks like this. Notice that there are only two lines.

1           0 BUILD_LIST               0
            2 RETURN_VALUE

You can inspect the bytecode for list() as follows.

from dis import dis

dis('list()')

The bytecode looks like this. Notice that there are three lines.

1           0 LOAD_NAME                0 (list)
            2 CALL_FUNCTION            0
            4 RETURN_VALUE

Here’s the bytecode for {}.

1           0 BUILD_MAP                0
            2 RETURN_VALUE

Here’s the bytecode for dict().

1           0 LOAD_NAME                0 (dict)
            2 CALL_FUNCTION            0
            4 RETURN_VALUE