I want to understand from which point a Python program starts running. I have previous experience in Java. In Java every program starts from main() function of it's Main class. Knowing this I can determine the execution sequence of other classes or functions of other Classes. I know that in Python I can control program execution sequence by using __name__ like this:
def main():
print("This is the main routine.")
if __name__ == "__main__":
main()
But when we don't use __name__ then what is the starting line of my Python program?
Interpreter starts to interpret file line by line from the beginning.
If it encounters function definition, it adds it into the globals
dict. If it encounters function call, it searches it in globals
dict and executes or fail.
# foo.py
def foo():
print "hello"
foo()
def test()
print "test"
print "global_string"
if __name__ == "__main__":
print "executed"
else:
print "imported"
Output
hello
global_string
executed
Interpreter starts to interpret foo.py line by line from the beginning like first the function definition it adds to globals dict and then it encounters the call to the function foo() and execute it so it prints hello.
After that, it adds test() to global dict but there's no function call to this function so it will not execute the function.
After that print statement will execute will print global_string.
After that, if condition will execute and in this case, it matched and will print executed.
Related
Why is it standard in Python to have the main() function and the if __name__ == '__main__' check at the end of the block of code? It also seems standard for the abstraction of the functions to follow the same pattern upwards. What I mean is that the definition of the function to be executed by main() is above the main() and the definition of the functions inside that are above and so on..
That seems odd because when one opens the module to read the code, it ends up starting with low-level code and moves up to higher level functions. Isn't it hard to grasp what the module is doing that way?
Why not do the alternative? Have the if __name__ check at the top followed by the main() function, and so on. This way, one quickly glances at what the main() function does and understands what the code is about.
You can't call main() before it is defined. And main cannot call other functions before they are defined.
Example 1:
if __name__=='__main__':
main()
def main():
print("Hello")
This will error because main hasn't been defined yet at the point where you try and execute it.
Example 2:
def main():
hello()
if __name__=='__main__':
main()
def hello():
print("Hello")
This will error because main() is executed and tries to call hello before it is defined.
The if __name__=='__main__': which contains the call to main() works best at the end of the file so that everything that it needs has been defined before it is reached.
Where you put the main definition itself is more flexible, but putting it at the end (just before the if __name__=='__main__': block that calls it) makes as much sense as anywhere else.
The purpose of an if __name__ == '__main__': guard is to prevent a module from having side-effects when it's imported.
The fact that it is a module rather than a script implies that it will normally be used by other code importing the definitions from it, not executed directly. So given that, it makes sense that the functions, classes and constants appear first in the source code, since those are what users of the module will be importing (and hence, what they might want to see the source of).
So even if the code guarded by if __name__ == '__main__': doesn't rely on the definitions in the module already having been evaluated (which would be unusual), this part of the code is usually the least important to the users of the module, so it doesn't belong at the start of the file.
I have this code:
import sys
def random(size=16):
return open(r"C:\Users\ravishankarv\Documents\Python\key.txt").read(size)
def main():
key = random(13)
print(key)
When I try running the script, there are no errors, but nothing appears to happen. I expected it to print some content from the key file, but nothing is printed.
What is wrong? How do I make the code run?
You've not called your main function at all, so the Python interpreter won't call it for you.
Add this as the last line to just have it called at all times:
main()
Or, if you use the commonly seen:
if __name__ == "__main__":
main()
It will make sure your main method is called only if that module is executed as the starting code by the Python interpreter. More about that here: What does if __name__ == "__main__": do?
If you want to know how to write the best possible 'main' function, Guido van Rossum (the creator of Python) wrote about it here.
Python isn't like other languages where it automatically calls the main() function. All you have done is defined your function.
You have to manually call your main function:
main()
Also, you may commonly see this in some code:
if __name__ == '__main__':
main()
There's no such main method in python, what you have to do is:
if __name__ == '__main__':
main()
Something does happen, it just isn't noticeable
Python runs scripts from top to bottom. def is a statement, and it executes when it is encountered, just like any other statement. However, the effect of this is to create the function (and assign it a name), not to call it. Similarly, import is a statement that loads the other module (and makes its code run top to bottom, with its own global-variable context), and assigns it a name.
When the example code runs, therefore, three things happen:
The code for the sys standard library module runs, and then the name sys in our own module's global variables is bound to that module
A function is created from the code for random, and then the name random is bound to that function
A function is created from the code for main, and then the name main is bound to that function
There is nothing to call the functions, so they aren't called. Since they aren't called, the code inside them isn't run - it's only used to create the functions. Since that code doesn't run, the file isn't read and nothing is printed.
There are no "special" function names
Unlike in some other languages, Python does not care that a function is named main, or anything else. It will not be run automatically.
As the Zen of Python says, "Explicit is better than implicit". If we want a function to be called, we have to call it. The only things that run automatically are the things at top level, because those are the instructions we explicitly gave.
The script starts at the top
In many real-world scripts, you may see a line that says if __name__ == '__main__':. This is not "where the script starts". The script runs top to bottom.
Please read What does if __name__ == "__main__": do? to understand the purpose of such an if statement (short version: it makes sure that part of your top-level code is skipped if someone else imports this file as a module). It is not mandatory, and it does not have any kind of special "signalling" purpose to say where the code starts running. It is just a perfectly normal if statement, that is checking a slightly unusual condition. Nothing requires you to use it in a script (aside from wanting to check what it checks), and nothing prevents you from using it more than once. Nothing prevents you from checking whether __name__ is equal to other values, either (it's just... almost certainly useless).
You're not calling the function. Put main() at the bottom of your code.
This question already has answers here:
Why doesn't the main() function run when I start a Python script? Where does the script start running (what is its entry point)?
(5 answers)
Closed 6 years ago.
I am writing a simple Python program with some functions, one of which is a main() function executes the other functions. However when I run the code below there is no output. Can someone tell me if they see an error in the structure?
def print1():
print("this is also a function")
def print2():
print("this is a function")
def main():
print1()
print2()
You need to call main(). Right now it is just a definition. What use is an entry in a dictionary if nobody uses the word?
def print1():
print("this is also a function")
def print2():
print("this is a function")
def main():
print1()
print2()
main()
It is common in Python programs to do things differently depending on if the file is being imported or run. When a file is executed, the __name__ variable is set either to '__main__' or the name of the file. It is set to '__main__' if the file is being executed as a python script, and it is set to the name of the file if it is being imported. You can use this information so that you don't actually run anything if it is just being imported instead of being run as a python script:
if __name__ == '__main__':
main()
That way, you can import the module, and use the functions without main() being called. If it is run as a python script, however, main() will be called.
Add this to the bottom of your code.
if __name__ == "__main__":
main()
See https://docs.python.org/2/library/main.html
Main needs to be called explicitly. You can do it without the if statement, but this allows your code to be either a module or a main program. If it is imported as a module, main() won't be called. If it is the main program then it will be called.
You are thinking like a C programmer. In this case python acts more like a shell script. Anything not in a function or class definition will be executed.
You need to call main() in order for it to run.
I believe what you mean to be doing is
def print1():
print("this is also a function")
def print2():
print("this is a function")
if __name__ == '__main__':
print1()
print2()
Call this script something.py and then run python something.py from your command line.
So in my file1.py, I have something like:
def run():
# Do something
print "Hi"
Now I want to use function run() in another file.
from file1.py import run
However, when I execute the other file it also prints Hi. How do I suppress this?
Add the print "Hi" in an if __name__ == "__main__" clause.
When python imports modules it executes the code contained in them in order to build the module namespace. If you run the module as the main script the __name__ is going to get assigned to __main__ and the code inside the if clause is going to get executed.
Since you're not running the script as the main script the __name__ gets assigned to the modules __name__ (in this case file1) and as a result this test will not succeed and the print statement is not going to get executed.
def run():
# Do something
if __name__ == "__main__":
print "Hi"
You should include after the functions this:
this runs the program
if main == "name":# before and after 'main' and 'name' there are two under_scores!
print "hi" etc...
if you don't want Hi to be printed, simply delete from your file1.py
if you want Hi to be printed when run() is called, then indent it so that it belongs to the run() function.
I have this code:
import sys
def random(size=16):
return open(r"C:\Users\ravishankarv\Documents\Python\key.txt").read(size)
def main():
key = random(13)
print(key)
When I try running the script, there are no errors, but nothing appears to happen. I expected it to print some content from the key file, but nothing is printed.
What is wrong? How do I make the code run?
You've not called your main function at all, so the Python interpreter won't call it for you.
Add this as the last line to just have it called at all times:
main()
Or, if you use the commonly seen:
if __name__ == "__main__":
main()
It will make sure your main method is called only if that module is executed as the starting code by the Python interpreter. More about that here: What does if __name__ == "__main__": do?
If you want to know how to write the best possible 'main' function, Guido van Rossum (the creator of Python) wrote about it here.
Python isn't like other languages where it automatically calls the main() function. All you have done is defined your function.
You have to manually call your main function:
main()
Also, you may commonly see this in some code:
if __name__ == '__main__':
main()
There's no such main method in python, what you have to do is:
if __name__ == '__main__':
main()
Something does happen, it just isn't noticeable
Python runs scripts from top to bottom. def is a statement, and it executes when it is encountered, just like any other statement. However, the effect of this is to create the function (and assign it a name), not to call it. Similarly, import is a statement that loads the other module (and makes its code run top to bottom, with its own global-variable context), and assigns it a name.
When the example code runs, therefore, three things happen:
The code for the sys standard library module runs, and then the name sys in our own module's global variables is bound to that module
A function is created from the code for random, and then the name random is bound to that function
A function is created from the code for main, and then the name main is bound to that function
There is nothing to call the functions, so they aren't called. Since they aren't called, the code inside them isn't run - it's only used to create the functions. Since that code doesn't run, the file isn't read and nothing is printed.
There are no "special" function names
Unlike in some other languages, Python does not care that a function is named main, or anything else. It will not be run automatically.
As the Zen of Python says, "Explicit is better than implicit". If we want a function to be called, we have to call it. The only things that run automatically are the things at top level, because those are the instructions we explicitly gave.
The script starts at the top
In many real-world scripts, you may see a line that says if __name__ == '__main__':. This is not "where the script starts". The script runs top to bottom.
Please read What does if __name__ == "__main__": do? to understand the purpose of such an if statement (short version: it makes sure that part of your top-level code is skipped if someone else imports this file as a module). It is not mandatory, and it does not have any kind of special "signalling" purpose to say where the code starts running. It is just a perfectly normal if statement, that is checking a slightly unusual condition. Nothing requires you to use it in a script (aside from wanting to check what it checks), and nothing prevents you from using it more than once. Nothing prevents you from checking whether __name__ is equal to other values, either (it's just... almost certainly useless).
You're not calling the function. Put main() at the bottom of your code.