python – What is the preferred syntax for initializing a dict: curly brace literals {} or the dict() function?

python – What is the preferred syntax for initializing a dict: curly brace literals {} or the dict() function?

Curly braces. Passing keyword arguments into dict(), though it works beautifully in a lot of scenarios, can only initialize a map if the keys are valid Python identifiers.

This works:

a = {import: trade, 1: 7.8}

a = dict({import: trade, 1: 7.8})

This wont work:

a = dict(import=trade, 1=7.8)

It will result in the following error:

    a = dict(import=trade, 1=7.8)
             ^
SyntaxError: invalid syntax

The first, curly braces. Otherwise, you run into consistency issues with keys that have odd characters in them, like =.

# Works fine.
a = {
    a: value,
    b=c: value,
}

# Eeep! Breaks if trying to be consistent.
b = dict( 
    a=value,
    b=c=value,
)

python – What is the preferred syntax for initializing a dict: curly brace literals {} or the dict() function?

The first version is preferable:

• It works for all kinds of keys, so you can, for example, say {1: one, 2: two}. The second variant only works for (some) string keys. Using different kinds of syntax depending on the type of the keys would be an unnecessary inconsistency.

• It is faster:

  $ python -m timeit dict(a=value, another=value)
  1000000 loops, best of 3: 0.79 usec per loop
  $ python -m timeit {a: value,another: value}
  1000000 loops, best of 3: 0.305 usec per loop
  

• If the special syntax for dictionary literals wasnt intended to be used, it probably wouldnt exist.