4.8. A Python program¶
Here is the Python program
hello.py first discussed in hello_world.
If you didn’t do it the first time we discussed this
program, you should now download it and put it in a file called
As discussed in that section, the file hello.py is a Python file that contains an extremely simple Python program. In this section we discuss what makes it a script, that is, a Python program that can be run from the command line by connecting the directory in which you saved hello.py and typing:
$ python hello.py
$ is my commandline prompt; the rest
is what I typed on my keyboard, followed by
If Python is running and you call the program this way,
The following is what the file contains:
1import sys 2 3def main(): 4 """ 5 Get the name from the command line, using 'World' as a fallback. 6 """ 7 if len(sys.argv) >= 2: 8 name = sys.argv 9 else: 10 name = 'World' 11 print ('Hello', name) 12 13if __name__ == '__main__': 14 main()
Consider the somewhat mysterious looking last two lines. What exactly do they do?
To answer this question, let’s delete the last two
lines of the program, and save the result in a new file
truncated_hello.py. Now run this new truncated
file from the commandline just as you ran hello.py:
$ python truncated_hello.py
What happens? What happens is that the program now does nothing. No hello. So those two lines were pretty important.
One more experiment. Try just typing
bring up the Python prompt. Next import
Still nothing happens. Now try running the program just
as in the last line of hello.py:
>>> import hello >>> hello.main() Hello, World
The Hello, World came back.
So what happened?
What happens when you import a file or just call it with Python as in:
$ python hello.py
is that every instruction in the program gets executed. In the
hello.py, that includes
all the function definitions and the
if statement at the end
of the file:
if __name__ == '__main__': main()
As with any
if statement, the second line
if statement is not run if the
condition on the first line is not met. The condition
__name__ == '__main__' is
True only if hello.py is being called on
the Python commandline. When hello.py is imported
it is false. Thus the last line of the file only
gets executed when hello.py is being called on
the Python commandline (which we will refer to
as using hello.py as a script).
In both importing and executing from the commandline
all of the file is executed, but the value of
is different in the two cases, and different things happen when
the file is loaded.
The bottom line: if you want to define a set of instructions to be executed when the file is run as a script (from the commandline), indent them under an:
if ___name__ == '__main__':