Today I learned that the module builtins grows dynamically in the REPL.

The appearing built-in

After some back and forth on BlueSky, I tripped on some funky behaviour in the REPL:

# Fresh new REPL, Python 3.13:
>>> import builtins
>>> len(dir(builtins))
159
>>> len(dir(builtins))
160
>>> len(dir(builtins))
160

What the heck is going on? Why did the module builtins grow in size between the two calculations?!

I restarted the REPL and tried something else:

# Fresh new REPL, Python 3.13:
>>> import builtins
>>> print(dir(builtins))
[ ... ]
>>> print(dir(builtins))
[ ... ]

I printed the contents of the module builtins twice in a row and they were exactly the same... Then, I checked, and the length of the module builtins was still 159:

>>> len(dir(builtins))
159

But then I checked again, and it grew again:

>>> len(dir(builtins))
160

For a second I was left wondering what sort of magic trick the built-in len must be playing, since it looks like it's making the module builtins grow another member... But then I printed the contents of the module again and I found the culprit after using a text diff tool online:

That's when it hit me: the REPL has a special name _ that refers to the result of the last operation.

Importing the module isn't enough to make it show up because it's a statement and statements don't produce a value. Similarly, the code print(dir(builtins)) also isn't enough because printing “doesn't return a result”, but when I run len(dir(builtins)), the length is saved in _.

But why is this variable in the module builtins if other variables aren't..?

I am not entirely sure, but my guess is that this special _ isn't a regular variable! You can assign to _ and it will “overwrite” the other value:

# Fresh new REPL:
>>> 1 + 2
3
>>> _
3
>>> _ = 73
>>> print(_)
73

But it's not really overwriting the previous value... It's actually shadowing the other one! Here's proof:

# Fresh new REPL:
>>> 1 + 2
3
>>> _
3
>>> _ = 73
>>> print(_)
73
>>> import builtins
>>> builtins._
3
>>> globals()["_"]
73

Fun, right?

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