'exit' is not a keyword in Python, but no error occurs while using it

Question

I learn that exit is not a keyword in Python by,

import keyword
print('exit' in keyword.kwlist) # Output: False

But there is no reminder of NameError: name 'exit' is not defined while using it. The output of the following snippet code makes me confused. Can anyone help me out?

for i in range(5):
 print(i)
 cur=i if i<2 else exit
print(cur)
# Output
0
1
2
3
4
Use exit() or Ctrl-D (i.e. EOF) to exit

I am unable to get related info about exit from Python documentations, except for exit([code=None]).

• SparkAndShine: @Ian, it seems that exit equals Use exit() or Ctrl-D (i.e. EOF) to exit, like a constant string.
• Ian: Which is an interesting behavior, I suppose... like when you use console.. :)
• cdarke: You found docs.python.org/3.5/library/constants.html which says exit is a constant added by site.py, not a keyword or built-in function. Not a duplicate, but see stackoverflow.com/questions/6501121/…

Answer 1

Keywords are part of the python syntax. They usually have special meaning in statements (e.g. for, del, if ...). This has other consequences -- e.g. you can't make a variable with the same name as a keyword.

builtins are callable objects (e.g. functions or at least function-like) that python provides in the namespace by default. examples of builtin functions are things like sorted, id, vars, ...

It's worth noting that exit is a convenience provided when in an interactive session. It's highly encouraged to use sys.exit instead.

• cdarke: The python doc. calls exit a constant: docs.python.org/3.5/library/constants.html
• mgilson: @cdarke -- Yeah. I'm guessing that's because they didn't have a better thing to call it. It's not a builtin (don't want people using it in scripts because it isn't there ...). With that said -- calling it a constant is a little weird in my opinion...
• cdarke: Agreed, python doesn't really have constants (OK, readonly properties), and it breaks PEP008 if it is one because it should be uppercase!

Answer 2

exit is an instance of the Quitter class. The Quitter class defines an __repr__ method that returns the string that you see when you type exit into the shell. It also defines a __call__ method. Just as __init__ is called when you use a class like a function, __call__ is called when an instance is used like a function. Therefore, exit() calls the __call__ method, which exits the program.

Answer 3

exit is an Built-in Constants added by the site module.

The site module (which is imported automatically during startup, except if the -S command-line option is given) adds several constants to the built-in namespace. They are useful for the interactive interpreter shell and should not be used in programs.

Answer 4

exit is the sys.exit function when you are using the interactive console.

Many things exist while they are not keywords (e.g. sum, int...). So you can bind to existing names, but not to keywords

• cdarke: exit has the same functionality, but it is not the same as sys.exit. See @zondo's post and stackoverflow.com/questions/6501121/…