What is this odd colon behavior doing?

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You have accidentally written a syntactically correct variable annotation. That feature was introduced in Python 3.6 (see PEP 526).

Although a variable annotation is parsed as part of an annotated assignment, the assignment statement is optional:

annotated_assignment_stmt ::=  augtarget ":" expression ["=" expression]

Thus, in context["a"]: 2

  • context["a"] is the annotation target
  • 2 is the annotation itself
  • context["a"] is left uninitialised

The PEP states that “the target of the annotation can be any valid single assignment target, at least syntactically (it is up to the type checker what to do with this)”, which means that the key doesn’t need to exist to be annotated (hence no KeyError). Here’s an example from the original PEP:

d = {}
d['a']: int = 0  # Annotates d['a'] with int.
d['b']: int      # Annotates d['b'] with int.

Normally, the annotation expression should evaluate to a Python type —
after all the main use of annotations is type hinting, but it is not enforced. The annotation can be any valid Python expression, regardless of the type or value of the result.

As you can see, at this time type hints are very permissive and rarely useful, unless you have a static type checker such as mypy.

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