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

问题

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

回答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.

回答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.

回答3:

On my simple test,
Cell 1
a = 3
Cell 2
exit
cell 3
print(a)

resulted in

---------------------------------------------------------------------------
NameError                                 Traceback (most recent call last)
<ipython-input-1-3f786850e387> in <module>
----> 1 a

NameError: name 'a' is not defined

exit just kills the kernel that the notebook is relying on for execution.

Interestingly enough however, There seems to be a parameter you can pass to modify that behaviour as well.

Test 2: Cell 1
a = 3
Cell 2
exit(keep_kernel=True)
cell 3
print(a) resulted in 3

EDIT: And looks like @user2357112's answer fills in the missing pieces.
EDIT2: Actually, it seems to be an instance of IPython.core.autocall.ZMQExitAutocall

 class IPython.core.autocall.ZMQExitAutocall(ip=None)

    Bases: IPython.core.autocall.ExitAutocall

    Exit IPython. Autocallable, so it needn’t be explicitly called.
    Parameters: keep_kernel (bool) – If True, leave the kernel alive. Otherwise, tell the kernel to exit too (default).

来源：https://stackoverflow.com/questions/53676034/what-does-exit-keyword-do-in-python3-with-jupyter-notebook

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