[Python-ideas] PEP 550 dumbed down

Jim J. Jewett jimjjewett at gmail.com
Mon Aug 21 17:05:46 EDT 2017

I know I'm not the only one who is confused by at least some of the
alternative terminology choices.  I suspect I'm not the only one who
sometimes missed part of the argument because I was distracted
figuring out what the objects were, and forgot to verify what was
being done and why.  I also suspect that it could be much simpler to
follow if the API were designed in the abstract, with the
implementation left for later.

So is the following API missing anything important?

(1)  Get the current (writable) context.
    Currently proposed as a sys.* call, but I think injecting to
__builtins__ or globals would work as well.
(2)  Get a value from the current context, by string key.
    Currently proposed as key.get, rather env.__getitem__
(3)  Write a value to the current context, by string key.
    Currently proposed as key.set, rather env.__setitem__
(4)  Create a new (writable) empty context.
(5)  Create a copy of the current context, so that changes can be
isolated.  The copy will not be able to change anything in the current
context, though it can shadow keys.
(6)  Choose which context to use when calling another

At this point, it looks an awful lot like a subset of ChainMap, except that:

(A)  The current mapping is available through a series of sys.* calls.
(why not a builtin?  Or at least a global, injected when a different
environment is needed?)

(B)  Concurrency APIs are supposed to ensure that each
process/thread/Task/worker is using its own private context, unless
the call explicitly requests a shared or otherwise different context.

(C)  The current API requires users to initialize every key before it
can be added to a context.  This is presumably to support limits of
the proposed implementation.

If the semantics are right, and collections.ChainMap is rejected only
for efficiency, please say so in the PEP.  If the semantics are wrong,
please explain how they differ.

Sample code:

env["reason"] = "Spanish Inquisition"

with env.copy():
    env["username"] = "secret admin"
    print ("debugging", env["foodebug"])

with env.empty():
    assert "username" not in env

assert env["username"] is olduser


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