[Python-Dev] PEP 567 v2 (original) (raw)

Yury Selivanov yselivanov.ml at gmail.com
Wed Jan 3 00:05:11 EST 2018

On Wed, Jan 3, 2018 at 2:36 AM Victor Stinner <victor.stinner at gmail.com> wrote:

> I would really like to invite more people to review this PEP! I expect I'll be accepting it in the next two weeks, but it needs to go through more rigorous review.

I read again the PEP and I am still very confused by Context.run(). The PEP states multiple times that a context is immutable: * "read-only mapping" * inherit from Mapping, not from MutableMapping

But run() does modify the context (or please correct me if I completely misunderstood the PEP! I had to read it 3 times to check if run() mutates or not the context).

It would help if the ctx.run() example in the PEP would not only test var.get() but also test ctx.get(var). Or maybe show that the variable value is kept in a second function call, but the variable is "restored" between run() calls. The PEP tries hard to hide "context data", which is the only read only thing in the whole PEP, whereas it's a key concept to understand the implementation. I understood that: * ContextData is immutable * ContextVar.set() creates a new ContextData and sets it in the current Python thread state * When the called function completes, Context.run() sets its context data to the new context data from the Python thread state: so run() does modify the "immutable" context

tuples in Python are immutable, but you can have a tuple with a dict as its single element. The tuple is immutable, the dict is mutable.

At the C level we have APIs that can mutate a tuple though.

Now, tuple is not a direct analogy to Context, but there are some parallels. Context is a container like tuple, with some additional APIs on top.

The distinction between the internal/hiden immutable context data and public/visible "mutable" (from my point of view) context is unclear to me in the PEP. The concept of "current context" is not defined in the PEP. In practice, there is no "current context", there is only a "current context data" in the current Python thread. There is no need for a concrete context instance to store variable variables values. It's also hard to understand that in the PEP.

Why Context could not inherit from MutableMapping? (Allow ctx.set(var, value) and ctx [var] = value.) Is it just to keep the API small: changes should only be made using var.set()?

Because that would be confusing to end users.

ctx = copy_context() ctx[var] = something

What did we just do? Did we modify the 'var' in the code that is currently executing? No, you still need to call Context.run to see the new value for var.

Another problem is that MutableMapping defines a delitem method, which i don't want the Context to implement. Deleting variables like that is incompatible with PEP 550, where it's ambiguous (due to the stacked nature of contexts).

Now we don't want PEP 550 in 3.7, but I want to keep the door open for its design, in case we want context to work with generators.

Or maybe Context.run() should really be immutable and return the result of the called function and a new context? But I dislike such theorical API, since it would be complex to return the new context if the called function raises an exception.

It can't return a new context because the callable you're running can raise an exception. In which case you'd lose modifications prior to the error.

ps i'm on vacation and don't always have an internet connection.

yury

Victor

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