How are the contents of the builtins module available in the global namespace without import in Python?

Question

I've been using Python for a good period of time. I have never found out how built-in functions work. In different words, how are they included without having any module imported to use them? What if I want to add to them (locally)?

This may seem naive. But, I haven't really found any answer that explains comprehensively how do we have built-in functions, global variables, etc., available to us when developing a script.

In a nutshell, where do we include the builtins module?

I have encountered this question. But it gives a partial answer to my question.

Yes, they are part of the interpreter itself, not separated out into their own modules — UnholySheep, Jan 13 '19 at 22:06
You don't want to add to them (locally), except by defining non-built in functions. The last thing you want for readable code is functions that aren't in the standard globals but randomly appear anyway. — jonrsharpe, Jan 13 '19 at 22:07
@gilch: And there is [some (limited) precedent for dynamically adding stuff to the built-in namespace](https://docs.python.org/3/library/constants.html#constants-added-by-the-site-module). — ShadowRanger, Jan 13 '19 at 22:24
@ShadowRanger besides the standard library, I know that ipython and xonsh mess with builtins. — gilch, Jan 13 '19 at 22:28

user2357112 · Accepted Answer · 2019-01-13T22:46:05.140

The not-implementation-details part of the answer is that the builtins module, or __builtin__ in Python 2, provides access to the built-ins namespace. If you want to modify the built-ins (you usually shouldn't), setting attributes on builtins is how you'd go about it.

The implementation details part of the answer is that Python keeps track of built-ins in multiple ways. For example, each frame object keeps track of the built-in namespace it's using, which may be different from other frames' built-in namespaces. You can access this through a frame's f_builtins attribute. When a LOAD_GLOBAL instruction fails to find a name in the frame's globals, it looks in the frame's builtins. There's also a __builtins__ global variable in most global namespaces, but it's not directly used for built-in variable lookup; instead, it's used to initialize f_builtins in certain situations during frame object creation. There's also a builtins reference in the global PyInterpreterState, which is used as default builtins if there's no current frame object.

The docs for the `exec` function also mention that you can customize which `__builtins__` it uses by specifying it in the globals dict argument. — gilch, Jan 13 '19 at 22:26
@gilch: That's not *quite* what they say, but under usual circumstances, setting `__builtins__` in the global dict passed to `exec` will control the builtins used for the executed code, through the usual initialize-`f_builtins`-from-`__builtins__` handling. There is a weird exception [if you try to set `__builtins__` in the current globals and then pass those globals to `exec`](https://ideone.com/yl9fvN), since that hits the other code path for initializing `f_builtins`. — user2357112, Jan 13 '19 at 22:40

How are the contents of the builtins module available in the global namespace without import in Python?

1 Answers1