I have 2 python files. file1.py and file2.py
file1.py
print "file1"
file2.py
import file1
print "file2"
and when i run file2,the output is
file1
file2
The question may seem little naive but i want to know what exactly is happening here.
Thanks in Advance.
Yes.
When importing a file it is being run.
To avoid this, file1.py can be:
if __name__=='__main__':
print 'file1'
And then the text will be printed only if file1.py is the main file being run directly.
In a sense yes. When you import a file, you will run all the script and you will also initialize all the methods.
To ensure that code is only run when the file is run directly and not when it is imported. You should put all your main code in main() and do it as:
def main():
#all your main code here
if __name__ == '__main__':
main()
The import statement combines two operations; it searches for the named module, then it binds the results of that search to a name in the local scope. The search operation of the import statement is defined as a call to the import() function, with the appropriate arguments. The return value of import() is used to perform the name binding operation of the import statement. See the import statement for the exact details of that name binding operation.
import file using python
To print 'file2' your code, you need to pass it as a command to the Python interpreter,
python myscript.py
there's no main() function that gets run automatically, so the main() function is implicitly all the code at the top level, and call if __name__ == "__main__"
How main does in python
Related
I have a Python program I'm building that can be run in either of 2 ways: the first is to call python main.py which prompts the user for input in a friendly manner and then runs the user input through the program. The other way is to call python batch.py -file- which will pass over all the friendly input gathering and run an entire file's worth of input through the program in a single go.
The problem is that when I run batch.py, it imports some variables/methods/etc from main.py, and when it runs this code:
import main
at the first line of the program, it immediately errors because it tries to run the code in main.py.
How can I stop Python from running the code contained in the main module which I'm importing?
Because this is just how Python works - keywords such as class and def are not declarations. Instead, they are real live statements which are executed. If they were not executed your module would be empty.
The idiomatic approach is:
# stuff to run always here such as class/def
def main():
pass
if __name__ == "__main__":
# stuff only to run when not called via 'import' here
main()
It does require source control over the module being imported, however.
Due to the way Python works, it is necessary for it to run your modules when it imports them.
To prevent code in the module from being executed when imported, but only when run directly, you can guard it with this if:
if __name__ == "__main__":
# this won't be run when imported
You may want to put this code in a main() method, so that you can either execute the file directly, or import the module and call the main(). For example, assume this is in the file foo.py.
def main():
print "Hello World"
if __name__ == "__main__":
main()
This program can be run either by going python foo.py, or from another Python script:
import foo
...
foo.main()
Use the if __name__ == '__main__' idiom -- __name__ is a special variable whose value is '__main__' if the module is being run as a script, and the module name if it's imported. So you'd do something like
# imports
# class/function definitions
if __name__ == '__main__':
# code here will only run when you invoke 'python main.py'
Unfortunately, you don't. That is part of how the import syntax works and it is important that it does so -- remember def is actually something executed, if Python did not execute the import, you'd be, well, stuck without functions.
Since you probably have access to the file, though, you might be able to look and see what causes the error. It might be possible to modify your environment to prevent the error from happening.
Put the code inside a function and it won't run until you call the function. You should have a main function in your main.py. with the statement:
if __name__ == '__main__':
main()
Then, if you call python main.py the main() function will run. If you import main.py, it will not. Also, you should probably rename main.py to something else for clarity's sake.
There was a Python enhancement proposal PEP 299 which aimed to replace if __name__ == '__main__': idiom with def __main__:, but it was rejected. It's still a good read to know what to keep in mind when using if __name__ = '__main__':.
You may write your "main.py" like this:
#!/usr/bin/env python
__all__=["somevar", "do_something"]
somevar=""
def do_something():
pass #blahblah
if __name__=="__main__":
do_something()
I did a simple test:
#test.py
x = 1
print("1, has it been executed?")
def t1():
print("hello")
print("2, has it been executed?")
def t2():
print("world")
print("3, has it been executed?")
def main():
print("Hello World")
print("4, has it been executed?")
print("5, has it been executed?")
print(x)
# while True:
# t2()
if x == 1:
print("6, has it been executed?")
#test2.py
import test
When executing or running test2.py, the running result:
1, has it been executed?
5, has it been executed?
1
6, has it been executed?
Conclusion: When the imported module does not add if __name__=="__main__":, the current module is run, The code in the imported module that is not in the function is executed sequentially, and the code in the function is not executed when it is not called.
in addition：
def main():
# Put all your code you need to execute directly when this script run directly.
pass
if __name__ == '__main__':
main()
else:
# Put functions you need to be executed only whenever imported
A minor error that could happen (at least it happened to me), especially when distributing python scripts/functions that carry out a complete analysis, was to call the function directly at the end of the function .py file.
The only things a user needed to modify were the input files and parameters.
Doing so when you import you'll get the function running immediately. For proper behavior, you simply need to remove the inside call to the function and reserve it for the real calling file/function/portion of code
Another option is to use a binary environment variable, e.g. lets call it 'run_code'. If run_code = 0 (False) structure main.py to bypass the code (but the temporarily bypassed function will still be imported as a module). Later when you are ready to use the imported function (now a module) set the environment variable run_code = 1 (True). Use the os.environ command to set and retrieve the binary variable, but be sure to convert it to an integer when retrieving (or restructure the if statement to read a string value),
in main.py:
import os
#set environment variable to 0 (False):
os.environ['run_code'] = '0'
def binary_module():
#retrieve environment variable, convert to integer
run_code_val = int(os.environ['run_code'] )
if run_code_val == 0:
print('nope. not doing it.')
if run_code_val == 1:
print('executing code...')
# [do something]
...in whatever script is loading main.py:
import os,main
main.binary_module()
OUTPUT: nope. not doing it.
# now flip the on switch!
os.environ['run_code'] = '1'
main.binary_module()
OUTPUT: executing code...
*Note: The above code presumes main.py and whatever script imports it exist in the same directory.
Although you cannot use import without running the code; there is quite a swift way in which you can input your variables; by using numpy.savez, which stores variables as numpy arrays in a .npz file. Afterwards you can load the variables using numpy.load.
See a full description in the scipy documentation
Please note this is only the case for variables and arrays of variable, and not for methods, etc.
Try just importing the functions needed from main.py? So,
from main import SomeFunction
It could be that you've named a function in batch.py the same as one in main.py, and when you import main.py the program runs the main.py function instead of the batch.py function; doing the above should fix that. I hope.
I am writing a Python (3.5) module in which I'd like to make use of an existing Python module from an open source project. The module I want to import contains:
several functions
a if __name__ == '__main__': instruction
but does not contain a def main(args) function.
Because there is no actual main function, I cannot import it by means of import module and use it as module.main(). Although I did find options to separately execute it as script via the commands os.system() and subprocess.Popen(), I am actually looking for a way to make this call an integral part of my code.
I understand I can add the def main() part myself in the original code, but because it comes from an open source project, I am looking for ways to leave it untouched, so that I don't need to maintain it myself if it gets updated.
I have gone through other very similar questions, such as this and this that could not solve my issue. This answer gives me the feeling what I am trying to do is not trivial.
Is there any way to do this?
When you import a module for the first time, (as distinguished from importing a function), all code in that module executes. That means functions become defined, global variables become defined, etc. The reason we write an if __name__ == "__main__": block is so that when importing a module, that code does not execute (it will only execute if name == "main"). If you simply remove the if __name__ == "__main__": line and fix the indentation, that code will execute when you import the module. take this module hello.py for example:
def hello_world():
print("Hello world")
if __name__ == "__main__":
hello_world()
then if we import:
import hello
hello_world()
The code below will do the same thing as this case where the first module is again hello.py:
def hello_world():
print("hello world")
hello_world()
module to be executed:
import hello
I recommend you do not do it this way though, you really should just edit to include a main function.
I'm creating a Python module mymodule.py and I need to run a function on import. The function should not be run by the user, and is only necessary to initiate the module properly.
Since it's a module, this won't work:
if __name__ == '__main__':
_main()
I want to follow PEPs and I was simply wondering if there an equivalent of C's main() in a Python module?
...or if I should just write the initialisation code inline (not as a function), or call the function inline.
Code that is not indented into a function will be run as the file is loaded (once). That's probably what you want:
def some_function():
pass
def other_function():
pass
init_value = 0 # This code here is run immediately.
buffer = None
Running a function on import is not equivalent to C's main(). C's main() is run when executing the program.
Everything in a Python module which is top-level (i.e. not in a function) is executed when the module is imported. For example, if this is your module content:
def _on_import():
pass # do something
_on_import()
then _on_import() is executed when importing the module.
When your module looks like this:
def main():
pass # do something
if __name__ == '__main__':
main()
then main() is executed when running the python module as a script (e.g. if your module is in file foo.py and you run python foo.py). This is basically equivalent to C's main().
Yeah have like a top level function that is like a main, something like this:
def interact():
# Your code which handles all the rest of the functions
# For example
print('Welcome to this program!')
filename = input('Please enter the data source file: ')
load_data(filename)
......
And then at the bottom of your script do what you were doing:
if __name__ == '__main__':
interact()
if you want to import other files into the program, like mymodule.py, just do this:
from mymodule import *
Alternatively, you can test functions out like this:
if __name__ == "__main__":
print test_the_function(123, 456)
When mymodule is imported, the code is run as before, but when we get to the if statement, Python looks to see what name the module has. Since the module is imported, we know it by the name used when importing it, so __name__ is mymodule. Thus, the print statement is never reached.
The beautiful thing about python is that it doesn't work like Cs main(). You just start typing, and you've written your first program. The simplicity of python is what makes it so beautiful, I wouldn't try and compare it with C, it just won't work.
You can also check this website out, if has some useful information about the python main():
https://wiki.python.org/moin/Asking%20for%20Help/Do%20you%20need%20a%20int%20main()%20like%20you%20do%20in%20c%2B%2B
I have a Python program I'm building that can be run in either of 2 ways: the first is to call python main.py which prompts the user for input in a friendly manner and then runs the user input through the program. The other way is to call python batch.py -file- which will pass over all the friendly input gathering and run an entire file's worth of input through the program in a single go.
The problem is that when I run batch.py, it imports some variables/methods/etc from main.py, and when it runs this code:
import main
at the first line of the program, it immediately errors because it tries to run the code in main.py.
How can I stop Python from running the code contained in the main module which I'm importing?
Because this is just how Python works - keywords such as class and def are not declarations. Instead, they are real live statements which are executed. If they were not executed your module would be empty.
The idiomatic approach is:
# stuff to run always here such as class/def
def main():
pass
if __name__ == "__main__":
# stuff only to run when not called via 'import' here
main()
It does require source control over the module being imported, however.
Due to the way Python works, it is necessary for it to run your modules when it imports them.
To prevent code in the module from being executed when imported, but only when run directly, you can guard it with this if:
if __name__ == "__main__":
# this won't be run when imported
You may want to put this code in a main() method, so that you can either execute the file directly, or import the module and call the main(). For example, assume this is in the file foo.py.
def main():
print "Hello World"
if __name__ == "__main__":
main()
This program can be run either by going python foo.py, or from another Python script:
import foo
...
foo.main()
Use the if __name__ == '__main__' idiom -- __name__ is a special variable whose value is '__main__' if the module is being run as a script, and the module name if it's imported. So you'd do something like
# imports
# class/function definitions
if __name__ == '__main__':
# code here will only run when you invoke 'python main.py'
Unfortunately, you don't. That is part of how the import syntax works and it is important that it does so -- remember def is actually something executed, if Python did not execute the import, you'd be, well, stuck without functions.
Since you probably have access to the file, though, you might be able to look and see what causes the error. It might be possible to modify your environment to prevent the error from happening.
Put the code inside a function and it won't run until you call the function. You should have a main function in your main.py. with the statement:
if __name__ == '__main__':
main()
Then, if you call python main.py the main() function will run. If you import main.py, it will not. Also, you should probably rename main.py to something else for clarity's sake.
There was a Python enhancement proposal PEP 299 which aimed to replace if __name__ == '__main__': idiom with def __main__:, but it was rejected. It's still a good read to know what to keep in mind when using if __name__ = '__main__':.
You may write your "main.py" like this:
#!/usr/bin/env python
__all__=["somevar", "do_something"]
somevar=""
def do_something():
pass #blahblah
if __name__=="__main__":
do_something()
I did a simple test:
#test.py
x = 1
print("1, has it been executed?")
def t1():
print("hello")
print("2, has it been executed?")
def t2():
print("world")
print("3, has it been executed?")
def main():
print("Hello World")
print("4, has it been executed?")
print("5, has it been executed?")
print(x)
# while True:
# t2()
if x == 1:
print("6, has it been executed?")
#test2.py
import test
When executing or running test2.py, the running result:
1, has it been executed?
5, has it been executed?
1
6, has it been executed?
Conclusion: When the imported module does not add if __name__=="__main__":, the current module is run, The code in the imported module that is not in the function is executed sequentially, and the code in the function is not executed when it is not called.
in addition：
def main():
# Put all your code you need to execute directly when this script run directly.
pass
if __name__ == '__main__':
main()
else:
# Put functions you need to be executed only whenever imported
A minor error that could happen (at least it happened to me), especially when distributing python scripts/functions that carry out a complete analysis, was to call the function directly at the end of the function .py file.
The only things a user needed to modify were the input files and parameters.
Doing so when you import you'll get the function running immediately. For proper behavior, you simply need to remove the inside call to the function and reserve it for the real calling file/function/portion of code
Another option is to use a binary environment variable, e.g. lets call it 'run_code'. If run_code = 0 (False) structure main.py to bypass the code (but the temporarily bypassed function will still be imported as a module). Later when you are ready to use the imported function (now a module) set the environment variable run_code = 1 (True). Use the os.environ command to set and retrieve the binary variable, but be sure to convert it to an integer when retrieving (or restructure the if statement to read a string value),
in main.py:
import os
#set environment variable to 0 (False):
os.environ['run_code'] = '0'
def binary_module():
#retrieve environment variable, convert to integer
run_code_val = int(os.environ['run_code'] )
if run_code_val == 0:
print('nope. not doing it.')
if run_code_val == 1:
print('executing code...')
# [do something]
...in whatever script is loading main.py:
import os,main
main.binary_module()
OUTPUT: nope. not doing it.
# now flip the on switch!
os.environ['run_code'] = '1'
main.binary_module()
OUTPUT: executing code...
*Note: The above code presumes main.py and whatever script imports it exist in the same directory.
Although you cannot use import without running the code; there is quite a swift way in which you can input your variables; by using numpy.savez, which stores variables as numpy arrays in a .npz file. Afterwards you can load the variables using numpy.load.
See a full description in the scipy documentation
Please note this is only the case for variables and arrays of variable, and not for methods, etc.
Try just importing the functions needed from main.py? So,
from main import SomeFunction
It could be that you've named a function in batch.py the same as one in main.py, and when you import main.py the program runs the main.py function instead of the batch.py function; doing the above should fix that. I hope.
While importing a python script from another script I want the script code that is classically protected by
if __name__ == "__main__":
....
....
to be run, how can I get that code run?
What I am trying to do is from a python script, dynamically change a module then import an existing script which should see the changes made and run its __main__ code like it was directly invoked by python?
I need to execute the 2nd python script in the same namespace as the 1st python script and pass the 2nd script command line parameters. execfile() suggested below might work but that doesn't take any command line parameters.
I would rather not edit the 2nd script (external code) as I want the 1st script to be a wrapper around it.
If you can edit the file being imported, one option is to follow the basic principle of putting important code inside of functions.
# Your script.
import foo
foo.main()
# The file being imported.
def main():
print "running foo.main()"
if __name__ == "__main__":
main()
If you can't edit the code being imported, execfile does provide a mechanism for passing arguments to the imported code, but this approach would make me nervous. Use with caution.
# Your script.
import sys
import foo
bar = 999
sys.argv = ['blah', 'fubb']
execfile( 'foo.py', globals() )
# The file being exec'd.
if __name__ == "__main__":
print bar
print sys.argv