python '__main__' - What does if __name__==“__main__”:do?
invalid syntax (23)
Let's look at the answer in a more abstract way:
Suppose we have this code in x.py:
... <Block A> if __name__ == '__main__': <Block B> ...
Blocks A and B are run when we are running "x.py".
But just block A (and not B) is run when we are running another module, "y.py" for example, in which x.y is imported and the code is run from there (like when a function in "x.py" is called from y.py).
What does the
if __name__ == "__main__": do?
# Threading example import time, thread def myfunction(string, sleeptime, lock, *args): while True: lock.acquire() time.sleep(sleeptime) lock.release() time.sleep(sleeptime) if __name__ == "__main__": lock = thread.allocate_lock() thread.start_new_thread(myfunction, ("Thread #: 1", 2, lock)) thread.start_new_thread(myfunction, ("Thread #: 2", 2, lock))
If this .py file are imported by other .py files, the code under "the if statement" will not be executed.
If this .py are run by
python this_py.py under shell, or double clicked in Windows. the code under "the if statement" will be executed.
It is usually written for testing.
I've been reading so much throughout the answers on this page. I would say, if you know the thing, for sure you will understand those answers, otherwise, you are still confused.
To be short, you need to know several points:
import aaction actually runs all that can be ran in "a"
Because of point 1, you may not want everything to be run in "a" when importing it
To solve the problem in point 2, python allows you to put a condition check
__name__is an implicit variable in all .py modules; when a.py is imported, the value of
__name__of a.py module is set to its file name "a"; when a.py is run directly using "python a.py", which means a.py is the entry point, then the value of
__name__of a.py module is set to a string
Based on the mechanism how python sets the variable
__name__for each module, do you know how to achieve point 3? The answer is fairly easy, right? Put a if condition:
if __name__ == "__main__": ...; you can even put if
__name__ == "a"depending on your functional need
The important thing that python is special at is point 4! The rest is just basic logic.
This answer is for Java programmers learning Python. Every Java file typically contains one public class. You can use that class in two ways:
Call the class from other files. You just have to import it in the calling program.
Run the class stand alone, for testing purposes.
For the latter case, the class should contain a public static void main() method. In Python this purpose is served by the globally defined label
When the Python interpreter reads a source file, it executes all of the code found in it.
Before executing the code, it will define a few special variables. For example, if the Python interpreter is running that module (the source file) as the main program, it sets the special
__name__ variable to have a value
"__main__". If this file is being imported from another module,
__name__ will be set to the module's name.
In the case of your script, let's assume that it's executing as the main function, e.g. you said something like
on the command line. After setting up the special variables, it will execute the
import statement and load those modules. It will then evaluate the
def block, creating a function object and creating a variable called
myfunction that points to the function object. It will then read the
if statement and see that
__name__ does equal
"__main__", so it will execute the block shown there.
One reason for doing this is that sometimes you write a module (a
.py file) where it can be executed directly. Alternatively, it can also be imported and used in another module. By doing the main check, you can have that code only execute when you want to run the module as a program and not have it execute when someone just wants to import your module and call your functions themselves.
See this page for some extra details.
Note (by Stainsor): If you put code before the function definitions, it will execute before main.
print("This code executes before main.") def functionA(): print("Function A") def functionB(): print("Function B") if __name__ == '__main__': functionA() functionB()
If this module is indeed main, this code results in:
This code executes before main. Function A Function B
If this module is not main, you get:
This code executes before main.
There are a number of variables that the system (Python interpreter) provides for source files (modules). You can get their values anytime you want, so, let us focus on the __name__ variable/attribute:
When Python loads a source code file, it executes all of the code found in it. (Note that it doesn't call all of the methods and functions defined in the file, but it does define them.)
Before the interpreter executes the source code file though, it defines a few special variables for that file; __name__ is one of those special variables that Python automatically defines for each source code file.
If Python is loading this source code file as the main program (i.e. the file you run), then it sets the special __name__ variable for this file to have a value "__main__".
If this is being imported from another module, __name__ will be set to that module's name.
So, in your example in part:
if __name__ == "__main__": lock = thread.allocate_lock() thread.start_new_thread(myfunction, ("Thread #: 1", 2, lock)) thread.start_new_thread(myfunction, ("Thread #: 2", 2, lock))
means that the code block:
lock = thread.allocate_lock() thread.start_new_thread(myfunction, ("Thread #: 1", 2, lock)) thread.start_new_thread(myfunction, ("Thread #: 2", 2, lock))
will be executed only when you run the module directly; the code block will not execute if another module is calling/importing it because the value of __name__ will not equal to "main" in that particular instance.
Hope this helps out.
if name == 'main':
We see if
__name__ == '__main__': quite often.
It checks if a module is being imported or not.
In other words, the code within the
if block will be executed only when the code runs directly. Here
Let's see what it does using a simple code that prints the name of the module:
# test.py def test(): print('test module name=%s' %(__name__)) if __name__ == '__main__': print('call test()') test()
If we run the code directly via
python test.py, the module name is
call test() test module name=__main__
Recently, i came across this while creating a class in python(deep learning) and to my understanding and in agreement to the top listed explanations above, i will elaborate
As sometimes we construct a class in a .py file, and defines a lot of functions inside that class. but we dont want to process all those class functions for a single purpose. forexample, for create a class and define some functions for data creation (.npy files) while some for data loading. so if we define
Means if we call the .py file, then it will only create data and will not process other class functions. other class functions (data loading) can also be imported by other classes.
What does the
if __name__ == "__main__":do?
To outline the basics:
The global variable,
__name__, in the module that is the entry point to your program, is
'__main__'. Otherwise, it's the name you import the module by.
So, code under the
ifblock will only run if the module is the entry point to your program.
It allows the code in the module to be importable by other modules, without executing the code block beneath on import.
Why do we need this?
Developing and Testing Your Code
Say you're writing a Python script designed to be used as a module:
def do_important(): """This function does something very important"""
You could test the module by adding this call of the function to the bottom:
and running it (on a command prompt) with something like:
~$ python important.py
However, if you want to import the module to another script:
On import, the
do_important function would be called, so you'd probably comment out your function call,
do_important(), at the bottom.
# do_important() # I must remember to uncomment to execute this!
And then you'll have to remember whether or not you've commented out your test function call. And this extra complexity would mean you're likely to forget, making your development process more troublesome.
A Better Way
__name__ variable points to the namespace wherever the Python interpreter happens to be at the moment.
Inside an imported module, it's the name of that module.
But inside the primary module (or an interactive Python session, i.e. the interpreter's Read, Eval, Print Loop, or REPL) you are running everything from its
So if you check before executing:
if __name__ == "__main__": do_important()
With the above, your code will only execute when you're running it as the primary module (or intentionally call it from another script).
An Even Better Way
There's a Pythonic way to improve on this, though.
What if we want to run this business process from outside the module?
If we put the code we want to exercise as we develop and test in a function like this and then do our check for
'__main__' immediately after:
def main(): """business logic for when running this module as the primary one!""" setup() foo = do_important() bar = do_even_more_important(foo) for baz in bar: do_super_important(baz) teardown() # Here's our payoff idiom! if __name__ == '__main__': main()
We now have a final function for the end of our module that will run if we run the module as the primary module.
It will allow the module and its functions and classes to be imported into other scripts without running the
main function, and will also allow the module (and its functions and classes) to be called when running from a different
'__main__' module, i.e.
import important important.main()
This module represents the (otherwise anonymous) scope in which the interpreter’s main program executes — commands read either from standard input, from a script file, or from an interactive prompt. It is this environment in which the idiomatic “conditional script” stanza causes a script to run:
if __name__ == '__main__': main()
Create a file, a.py:
print(__name__) # It will print out __main__
__name__ is always equal to
__main__ whenever that file is run directly showing that this is the main file.
Create another file, b.py, in the same directory:
import a # Prints a
Run it. It will print a, i.e., the name of the file which is imported.
So, to show two different behavior of the same file, this is a commonly used trick:
# Code to be run when imported into another python file if __name__ == '__main__': # Code to be run only when run directly
if __name__ == "__main__": main()
It checks if the
__name__ attribute of the Python script is
"__main__". In other words, if the program itself is executed, the attribute will be
__main__, so the program will be executed (in this case the
However, if your Python script is used by a module, any code outside of the
if statement will be executed, so
if \__name__ == "\__main__" is used just to check if the program is used as a module or not, and therefore decides whether to run the code.
All the answers have pretty much explained the functionality. But I will provide one example of its usage which might help clearing out the concept further.
Assume that you have two Python files, a.py and b.py. Now, a.py imports b.py. We run the a.py file, where the "import b.py" code is executed first. Before the rest of the a.py code runs, the code in the file b.py must run completely.
In the b.py code there is some code that is exclusive to that file b.py and we don't want any other file (other than b.py file), that has imported the b.py file, to run it.
So that is what this line of code checks. If it is the main file (i.e., b.py) running the code, which in this case it is not (a.py is the main file running), then only the code gets executed.
if __name__ == "__main__":do?
__name__ is a global variable (in Python, global actually means on the module level) that exists in all namespaces. It is typically the module's name (as a
As the only special case, however, in whatever Python process you run, as in mycode.py:
the otherwise anonymous global namespace is assigned the value of
'__main__' to its
Thus, including the final lines
if __name__ == '__main__': main()
- at the end of your mycode.py script,
- when it is the primary, entry-point module that is run by a Python process,
will cause your script's uniquely defined
main function to run.
Another benefit of using this construct: you can also import your code as a module in another script and then run the main function if and when your program decides:
import mycode # ... any amount of other code mycode.main()
There are lots of different takes here on the mechanics of the code in question, the "How", but for me none of it made sense until I understood the "Why". This should be especially helpful for new programmers.
Take file "ab.py":
def a(): print('A function in ab file'); a()
And a second file "xy.py":
import ab def main(): print('main function: this is where the action is') def x(): print ('peripheral task: might be useful in other projects') x() if __name__ == "__main__": main()
What is this code actually doing?
When you execute
import ab. The import statement runs the module immediately on import, so
ab's operations get executed before the remainder of
xy's. Once finished with
ab, it continues with
The interpreter keeps track of which scripts are running with
__name__. When you run a script - no matter what you've named it - the interpreter calls it
"__main__", making it the master or 'home' script that gets returned to after running an external script.
Any other script that's called from this
"__main__" script is assigned its filename as its
__name__ == "ab.py"). Hence, the line
if __name__ == "__main__": is the interpreter's test to determine if it's interpreting/parsing the 'home' script that was initially executed, or if it's temporarily peeking into another (external) script. This gives the programmer flexibility to have the script behave differently if it's executed directly vs. called externally.
Let's step through the above code to understand what's happening, focusing first on the unindented lines and the order they appear in the scripts. Remember that function - or
def - blocks don't do anything by themselves until they're called. What the interpreter might say if mumbled to itself:
- Open xy.py as the 'home' file; call it
- Import and open file with the
__name__ == "ab.py".
- Oh, a function. I'll remember that.
- Ok, function
a(); I just learned that. Printing 'A function in ab file'.
- End of file; back to
- Oh, a function. I'll remember that.
- Another one.
x(); ok, printing 'peripheral task: might be useful in other projects'.
- What's this? An
ifstatement. Well, the condition has been met (the variable
__name__has been set to
"__main__"), so I'll enter the
main()function and print 'main function: this is where the action is'.
The bottom two lines mean: "If this is the
"__main__" or 'home' script, execute the function called
main()". That's why you'll see a
def main(): block up top, which contains the main flow of the script's functionality.
Why implement this?
Remember what I said earlier about import statements? When you import a module it doesn't just 'recognize' it and wait for further instructions - it actually runs all the executable operations contained within the script. So, putting the meat of your script into the
main() function effectively quarantines it, putting it in isolation so that it won't immediately run when imported by another script.
Again, there will be exceptions, but common practice is that
main() doesn't usually get called externally. So you may be wondering one more thing: if we're not calling
main(), why are we calling the script at all? It's because many people structure their scripts with standalone functions that are built to be run independent of the rest of the code in the file. They're then later called somewhere else in the body of the script. Which brings me to this:
But the code works without it
Yes, that's right. These separate functions can be called from an in-line script that's not contained inside a
main() function. If you're accustomed (as I am, in my early learning stages of programming) to building in-line scripts that do exactly what you need, and you'll try to figure it out again if you ever need that operation again ... well, you're not used to this kind of internal structure to your code, because it's more complicated to build and it's not as intuitive to read.
But that's a script that probably can't have its functions called externally, because if it did it would immediately start calculating and assigning variables. And chances are if you're trying to re-use a function, your new script is related closely enough to the old one that there will be conflicting variables.
In splitting out independent functions, you gain the ability to re-use your previous work by calling them into another script. For example, "example.py" might import "xy.py" and call
x(), making use of the 'x' function from "xy.py". (Maybe it's capitalizing the third word of a given text string; creating a NumPy array from a list of numbers and squaring them; or detrending a 3D surface. The possibilities are limitless.)
(As an aside, this question contains an answer by @kindall that finally helped me to understand - the why, not the how. Unfortunately it's been marked as a duplicate of this one, which I think is a mistake.)
When your script is run by passing it as a command to the Python interpreter,
all of the code that is at indentation level 0 gets executed. Functions and classes that are defined are, well, defined, but none of their code gets run. Unlike other languages, there's no
main() function that gets run automatically - the
main() function is implicitly all the code at the top level.
In this case, the top-level code is an
__name__ is a built-in variable which evaluates to the name of the current module. However, if a module is being run directly (as in
myscript.py above), then
__name__ instead is set to the string
"__main__". Thus, you can test whether your script is being run directly or being imported by something else by testing
if __name__ == "__main__": ...
If your script is being imported into another module, its various function and class definitions will be imported and its top-level code will be executed, but the code in the then-body of the
if clause above won't get run as the condition is not met. As a basic example, consider the following two scripts:
# file one.py def func(): print("func() in one.py") print("top-level in one.py") if __name__ == "__main__": print("one.py is being run directly") else: print("one.py is being imported into another module")
# file two.py import one print("top-level in two.py") one.func() if __name__ == "__main__": print("two.py is being run directly") else: print("two.py is being imported into another module")
Now, if you invoke the interpreter as
The output will be
top-level in one.py one.py is being run directly
If you run
top-level in one.py one.py is being imported into another module top-level in two.py func() in one.py two.py is being run directly
Thus, when module
one gets loaded, its
"one" instead of
It is a special for when a Python file is called from the command line. This is typically used to call a "main()" function or execute other appropriate startup code, like commandline arguments handling for instance.
It could be written in several ways. Another is:
def some_function_for_instance_main(): dosomething() __name__ == '__main__' and some_function_for_instance_main()
I am not saying you should use this in production code, but it serves to illustrate that there is nothing "magical" about
if __name__ == '__main__'. It is a good convention for invoking a main function in Python files.
I think it's best to break the answer in depth and in simple words:
__name__: Every module in Python has a special attribute called
It is a built-in variable that returns the name of the module.
__main__: Like other programming languages, Python too has an execution entry point, i.e., main.
'__main__' is the name of the scope in which top-level code executes. Basically you have two ways of using a Python module: Run it directly as a script, or import it. When a module is run as a script, its
__name__ is set to
Thus, the value of the
__name__ attribute is set to
__main__ when the module is run as the main program. Otherwise the value of
__name__ is set to contain the name of the module.
You can make the file usable as a script as well as an importable module.
fibo.py (a module named
# Other modules can IMPORT this MODULE to use the function fib def fib(n): # write Fibonacci series up to n a, b = 0, 1 while b < n: print(b, end=' ') a, b = b, a+b print() # This allows the file to be used as a SCRIPT if __name__ == "__main__": import sys fib(int(sys.argv))
Before explaining anything about
if __name__ == '__main__' it is important to understand what
__name__ is and what it does.
It can not only be used in scripts but can also be found in both the interpreter and modules/packages.
>>> print(__name__) __main__ >>>
Module or package:
def somefunction(): print(__name__)
import somefile somefile.somefunction()
Notice that when used in a package or module,
__name__ takes the name of the file. The path of the actual module or package path is not given, but has its own DunderAlias
__file__, that allows for this.
You should see that, where
__name__, where it is the main file (or program) will always return
__main__, and if it is a module/package, or anything that is running off some other Python script, will return the name of the file where it has originated from.
Being a variable means that it's value can be overwritten ("can" does not mean "should"), overwriting the value of
__name__ will result in a lack of readability. So do not do it, for any reason. If you need a variable define a new variable.
It is always assumed that the value of
__name__ to be
__main__ or the name of the file. Once again changing this default value will cause more confusion that it will do good, causing problems further down the line.
>>> __name__ = 'Horrify' # Change default from __main__ >>> if __name__ == 'Horrify': print(__name__) ... >>> else: print('Not Horrify') ... Horrify >>>
It is considered good practice in general to include the
if __name__ == '__main__' in scripts.
Now to answer
if __name__ == '__main__':
Now we know the behaviour of
__name__ things become clearer:
if is a flow control statement that contains the block of code will execute if the value given is true. We have seen that
__name__ can take either
__main__ or the file name it has been imported from.
This means that if
__name__ is equal to
__main__ then the file must be the main file and must actually be running (or it is the interpreter), not a module or package imported into the script.
__name__ does take the value of
__main__ then whatever is in that block of code will execute.
This tells us that if the file running is the main file (or you are running from the interpreter directly) then that condition must execute. If it is a package then it should not, and the value will not be
__name__ can also be used in modules to define the name of a module
It is also possible to do other, less common but useful things with
__name__, some I will show here:
Executing only if the file is a module or package:
if __name__ != '__main__': # Do some useful things
Running one condition if the file is the main one and another if it is not:
if __name__ == '__main__': # Execute something else: # Do some useful things
You can also use it to provide runnable help functions/utilities on packages and modules without the elaborate use of libraries.
It also allows modules to be run from the command line as main scripts, which can be also very useful.
When you run Python interactively the local
__name__ variable is assigned a value of
__main__. Likewise, when you execute a Python module from the command line, rather than importing it into another module, its
__name__ attribute is assigned a value of
__main__, rather than the actual name of the module. In this way, modules can look at their own
__name__ value to determine for themselves how they are being used, whether as support for another program or as the main application executed from the command line. Thus, the following idiom is quite common in Python modules:
if __name__ == '__main__': # Do something appropriate here, like calling a # main() function defined elsewhere in this module. main() else: # Do nothing. This module has been imported by another # module that wants to make use of the functions, # classes and other useful bits it has defined.
The simplest explanation for the
__name__ variable (imho) is the following:
Create the following files.
# a.py import b
# b.py print "Hello World from %s!" % __name__ if __name__ == '__main__': print "Hello World again from %s!" % __name__
Running them will get you this output:
$ python a.py Hello World from b!
As you can see, when a module is imported, Python sets
globals()['__name__'] in this module to the module's name.
$ python b.py Hello World from __main__! Hello World again from __main__!
As you can see, when a file is executed, Python sets
globals()['__name__'] in this file to
The reason for
if __name__ == "__main__": main()
is primarily to avoid the import lock problems that would arise from having code directly imported. You want
main() to run if your file was directly invoked (that's the
__name__ == "__main__" case), but if your code was imported then the importer has to enter your code from the true main module to avoid import lock problems.
A side-effect is that you automatically sign on to a methodology that supports multiple entry points. You can run your program using
main() as the entry point, but you don't have to. While
main(), other tools use alternate entry points. For example, to run your file as a
gunicorn process, you define an
app() function instead of a
main(). Just as with
gunicorn imports your code so you don't want it do do anything while it's being imported (because of the import lock issue).
The type() function can return the type of an object or create a new type,
for example, we can create a Hi class with the type() function and do not need to use this way with class Hi(object):
def func(self, name='mike'): print('Hi, %s.' % name) Hi = type('Hi', (object,), dict(hi=func)) h = Hi() h.hi() Hi, mike. type(Hi) type type(h) __main__.Hi
In addition to using type() to create classes dynamically, you can control creation behavior of class and use metaclass.
According to the Python object model, the class is the object, so the class must be an instance of another certain class. By default, a Python class is instance of the type class. That is, type is metaclass of most of the built-in classes and metaclass of user-defined classes.
class ListMetaclass(type): def __new__(cls, name, bases, attrs): attrs['add'] = lambda self, value: self.append(value) return type.__new__(cls, name, bases, attrs) class CustomList(list, metaclass=ListMetaclass): pass lst = CustomList() lst.add('custom_list_1') lst.add('custom_list_2') lst ['custom_list_1', 'custom_list_2']
Magic will take effect when we passed keyword arguments in metaclass, it indicates the Python interpreter to create the CustomList through ListMetaclass. new (), at this point, we can modify the class definition, for example, and add a new method and then return the revised definition.