On: Diving into the world of hash tables

[rectangletangle]

When the name `a` is assigned the value of `baz`, Python is setting a key name `a` in the local dictionary (hash table) `locals()`.

Basically `a = 1` is syntactic sugar for `locals()['a'] = 1`

  >>> a
  Traceback (most recent call last):
    File "<stdin>", line 1, in <module>
  NameError: name 'a' is not defined
  >>> locals()['a'] = 113
  >>> a
  113
  >>>

One interesting side-effect of this is you can assign names that are not valid Python syntax explicitly.

For example:

  >>> locals()['foo-bar'] = 1
  >>> locals()['foo-bar']
  1
  >>> foo-bar
  Traceback (most recent call last):
    File "<stdin>", line 1, in <module>
  NameError: name 'foo' is not defined
  >>> 

The name `foo-bar` can't be literally referenced, because the interpreter attempts to interpret it as the subtraction operation `foo - bar`.

[adrian17 → rectangletangle]

This only works in global scope. It won't work in a function:

    def f():
        a = 5
        locals()['a'] = 6
        print(a) # 5

Inside a function, accesses/writes of locals use an internal array of local variables, to skip dicts. Same with constants. See:

https://github.com/python/cpython/blob/master/Python/ceval.c...

[hueving → rectangletangle]

locals() is presented to the user as a dictionary, but is that the way cpython actually works with it internally? I've run into weird GC issues that imply it's not a normal dictionary and it's just presented to the user as one for programattic access.

[klodolph → hueving]

No, that's not how it works internally. The Python compiler resolves all local variables at compile-time. You can see this for yourself by defining a simple function f, import dis, and calling dis.dis(f).

Global variables use a dictionary, however. The disassembly actually looks similar for both.

[rectangletangle → hueving]

That's a good question, I'm pretty sure it's not a normal dictionary. However, I'd have to go through the CPython source to confirm. Maybe someone who's more familiar with CPython's implementation will chime in.