[Python-Dev] PEP 567 pre v3 (original) (raw)

Victor Stinner victor.stinner at gmail.com
Mon Jan 8 17:35:40 EST 2018

Le 8 janv. 2018 8:36 PM, "Yury Selivanov" <yselivanov.ml at gmail.com> a écrit :

  1. Context.contains, Context.getitem and ContexVar.default

So if we keep the current PEP 567 behaviour w.r.t. defaults, ContextVar.get() might return a different value from Context.get():

v = ContextVar('v', default=42)
ctx = contextvars.copy_context()

ctx.get(v)   # returns None
v.get()   # returns 42
v in ctx  # returns False

I think this discrepancy is OK. Context is a mapping-like object and it reflects the contents of the underlying _ContextData mapping object.

ctx[var] raises an exception but ctx.get(var) returns None in such case. My point is just that Context.get() behaves differently than dict.get(). If dict[key] raises, I expect that dict.get() raises too and that I have to write explicitely dict.get(default=None).

I suggest to modify Context.get() to raise an exception or require to explicitely write ctx.get(var, default=None).

ContextVar.default is meant to be used only by ContextVar.get(). Context objects should not use it.

I now agree. The difference between ContextVar.get() and Context.get() is fine and can be explained.

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