Reply to this:
> How is that different from every other case of shadowing a builtin?
> 
> len = 45
> print(len("hello world"))

```
AssertionError = 42
assert 1 != 2
```

`assert` implicitly invokes `AssertionError`, while `len` does that explicitly.  That is to say, simply changing a global variable breaks the work of a keyword.

Another difference is that shadow builtins could be resumed in the 
nested functions without something like `globals()` or `exec(..., {})`, while you cannot perform this to the breakage of `assert`:

```

len = 1
def g():
   from builtins import len

   return len([1, 2, 3])

g() # => 3

AssertionError = +1

def f():
   from builtins import AssertionError
   assert False

f() # boooom


```