python - What is this odd colon behavior doing?

Question

I am using Python 3.6.1, and I have come across something very strange. I had a simple dictionary assignment typo that took me a long time to find.

context = {}
context["a"]: 2
print(context)

Output

{}

What is the code context["a"]: 2 doing? It doesn't raise a SyntaxError when it should IMO. At first I thought it was creating a slice. However, typing repr(context["a"]: 2) raises a SyntaxError. I also typed context["a"]: 2 in the console and the console didn't print anything. I thought maybe it returned None, but I'm not so sure.

I've also thought it could be a single line if statement, but that shouldn't be the right syntax either.

Additionally, context["a"] should raise a KeyError.

I am perplexed. What is going on?

Already this question has a dupe and it's pretty clear this is confusing for Python novices. I guess this is worst if Python is your only language, where type hinting and variable definition prior to initialisation in general might feel foreign. I imagine raising an error is impossible as this behaviour is deliberate and sometimes useful as explained in PEP 526, and you don't want to break compatibility. However, I wonder if the Python devs would consider adding a useful warning message for some cases. — Chris_Rands
Does this answer your question? What are variable annotations in Python 3.6? — Georgy

vaultah vaultah · Accepted Answer · 2018-01-18T14:27:50

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.

python - What is this odd colon behavior doing?

1 Answers