What does `exit` keyword do in Python3 with Jupyter Notebook?

Question

I am currently using Python3 in Jupyter Notebook and I just ran into a keyword exit. What does this keyword do ?

with open("some_file.txt") as f:
    for lines in f:
        print(lines)
        exit

Answer 1

The exit lines in your loop do nothing. Why they do nothing is a bit more complicated than the usual reason exit would do nothing in Python, though.

---

Normally, exit on a line by its own wouldn't exit Python. At most, in interactive mode, it would print a message telling you how to quit Python (message implemented in _sitebuiltins.Quitter.__repr__):

>>> exit
Use exit() or Ctrl-D (i.e. EOF) to exit

IPython does something different. Among the many extra systems IPython has for interactive convenience is a system to autocall instances of a certain type, IPython.core.autocall.IPyAutocall. (This is similar to but distinct from the %autocall magic.)

In IPython, exit and quit are set to instances of IPython.core.autocall.ExitAutocall, a subclass of IPyAutocall. IPython recognizes objects of this type, so when a line containing just exit or quit is executed, IPython actually exits.

In [1]: exit
[IPython dies here]

A Jupyter notebook's IPython kernel has exit and quit set to instances of the very closely related IPython.core.autocall.ZMQExitAutocall, which has some extra functionality to support a keep_kernel argument, but is otherwise the same.

This functionality only triggers when a line referring to the autocallable object is the entire content of the cell, though. Inside a loop, the autocall functionality doesn't trigger, so we're back to nothing happening.

In fact, even less happens than what would happen in normal interactive mode - in a normal, non-IPython interactive session, this loop would print the "Use exit()..." message on each iteration, due to differences in how IPython and the regular interactive mode handle expression auto-printing.

Answer 2

When exit (sic, with no parentheses) is used in iPython in a loop or a branch of a conditional statement, it is doing nothing because it is simply a reference to an instance of IPython.core.autocall.ExitAutocall:

for i in range(10): 
    exit 
print(i)
# 9

if i==9: 
   exit 
   print(exit)    
# <IPython.core.autocall.ExitAutocall object at 0x7f76ad78a4a8>      

It does not restart the kernel:

print(i)
# 9

However, when used on the command line alone, it is treated as a kind of magic (though without a %) and terminates the kernel.