Short Answer
It’s boilerplate code that protects users from accidentally invoking the script when they didn’t intend to. Here are some common problems when the guard is omitted from a script:
- If you import the guardless script in another script (e.g.
import my_script_without_a_name_eq_main_guard
), then the second script will trigger the first to run at import time and using the second script’s command line arguments. This is almost always a mistake. - If you have a custom class in the guardless script and save it to a pickle file, then unpickling it in another script will trigger an import of the guardless script, with the same problems outlined in the previous bullet.
Long Answer
To better understand why and how this matters, we need to take a step back to understand how Python initializes scripts and how this interacts with its module import mechanism.
Whenever the Python interpreter reads a source file, it does two things:
- it sets a few special variables like
__name__
, and then - it executes all of the code found in the file.
Let’s see how this works and how it relates to your question about the __name__
checks we always see in Python scripts.
Code Sample
Let’s use a slightly different code sample to explore how imports and scripts work. Suppose the following is in a file called foo.py
.
# Suppose this is foo.py. print("before import") import math print("before functionA") def functionA(): print("Function A") print("before functionB") def functionB(): print("Function B {}".format(math.sqrt(100))) print("before __name__ guard") if __name__ == '__main__': functionA() functionB() print("after __name__ guard")
Special Variables
When the Python interpreter reads a source file, it first defines a few special variables. In this case, we care about the __name__
variable.
When Your Module Is the Main Program
If you are running your module (the source file) as the main program, e.g.
python foo.py
the interpreter will assign the hard-coded string "__main__"
to the __name__
variable, i.e.
# It's as if the interpreter inserts this at the top # of your module when run as the main program. __name__ = "__main__"
When Your Module Is Imported By Another
On the other hand, suppose some other module is the main program and it imports your module. This means there’s a statement like this in the main program, or in some other module the main program imports:
# Suppose this is in some other main program. import foo
The interpreter will search for your foo.py
file (along with searching for a few other variants), and prior to executing that module, it will assign the name "foo"
from the import statement to the __name__
variable, i.e.
# It's as if the interpreter inserts this at the top # of your module when it's imported from another module. __name__ = "foo"
Executing the Module’s Code
After the special variables are set up, the interpreter executes all the code in the module, one statement at a time. You may want to open another window on the side with the code sample so you can follow along with this explanation.
Always
- It prints the string
"before import"
(without quotes). - It loads the
math
module and assigns it to a variable calledmath
. This is equivalent to replacingimport math
with the following (note that__import__
is a low-level function in Python that takes a string and triggers the actual import):
# Find and load a module given its string name, "math", # then assign it to a local variable called math. math = __import__("math")
- It prints the string
"before functionA"
. - It executes the
def
block, creating a function object, then assigning that function object to a variable calledfunctionA
. - It prints the string
"before functionB"
. - It executes the second
def
block, creating another function object, then assigning it to a variable calledfunctionB
. - It prints the string
"before __name__ guard"
.
Only When Your Module Is the Main Program
- If your module is the main program, then it will see that
__name__
was indeed set to"__main__"
and it calls the two functions, printing the strings"Function A"
and"Function B 10.0"
.
Only When Your Module Is Imported by Another
- (instead) If your module is not the main program but was imported by another one, then
__name__
will be"foo"
, not"__main__"
, and it’ll skip the body of theif
statement.
Always
- It will print the string
"after __name__ guard"
in both situations.
Summary
In summary, here’s what’d be printed in the two cases:
# What gets printed if foo is the main program before import before functionA before functionB before __name__ guard Function A Function B 10.0 after __name__ guard
# What gets printed if foo is imported as a regular module before import before functionA before functionB before __name__ guard after __name__ guard
Why Does It Work This Way?
You might naturally wonder why anybody would want this. Well, sometimes you want to write a .py
file that can be both used by other programs and/or modules as a module, and can also be run as the main program itself. Examples:
- Your module is a library, but you want to have a script mode where it runs some unit tests or a demo.
- Your module is only used as a main program, but it has some unit tests, and the testing framework works by importing
.py
files like your script and running special test functions. You don’t want it to try running the script just because it’s importing the module. - Your module is mostly used as a main program, but it also provides a programmer-friendly API for advanced users.
Beyond those examples, it’s elegant that running a script in Python is just setting up a few magic variables and importing the script. “Running” the script is a side effect of importing the script’s module.
Food for Thought
- Question: Can I have multiple
__name__
checking blocks? Answer: it’s strange to do so, but the language won’t stop you. - Suppose the following is in
foo2.py
. What happens if you saypython foo2.py
on the command-line? Why?
# Suppose this is foo2.py. import os, sys; sys.path.insert(0, os.path.dirname(__file__)) # needed for some interpreters def functionA(): print("a1") from foo2 import functionB print("a2") functionB() print("a3") def functionB(): print("b") print("t1") if __name__ == "__main__": print("m1") functionA() print("m2") print("t2")
- Now, figure out what will happen if you remove the
__name__
check infoo3.py
:
# Suppose this is foo3.py. import os, sys; sys.path.insert(0, os.path.dirname(__file__)) # needed for some interpreters def functionA(): print("a1") from foo3 import functionB print("a2") functionB() print("a3") def functionB(): print("b") print("t1") print("m1") functionA() print("m2") print("t2")
- What will this do when used as a script? When imported as a module?
# Suppose this is in foo4.py __name__ = "__main__" def bar(): print("bar") print("before __name__ guard") if __name__ == "__main__": bar() print("after __name__ guard")