Question

I just made a fresh copy of eclipse and installed pydev.

In my first trial to use pydev with eclipse, I created 2 module under the src package(the default one)

FirstModule.py:

'''
Created on 18.06.2009

@author: Lars Vogel
'''
def add(a,b):
    return a+b

def addFixedValue(a):
    y = 5
    return y +a

print "123"

run.py:

'''
Created on Jun 20, 2011

@author: Raymond.Yeung
'''
from FirstModule import add

print add(1,2)
print "Helloword"

When I pull out the pull down menu of the run button, and click “ProjectName run.py”, here is the result:

123
3
Helloword

Apparantly both module ran, why? Is this the default setting?

Answer 1

When you import a module, everything in it is “run”. This means that classes and function objects are created, global variables are set, and print statements are executed. *)

It is common practice to enclose statements only meant to be executed when the module is run directly in an if-block such as this:

if __name__ == "__main__":
    print "123"

Now if you run the module as a script, __name__ is set to "__main__", so "123" will be printed. However, if you import the module from somewhere else __name__ will be "FirstModule" in your case, not "__main__", so whatever is in the block will not be executed.

*) Note that if you import the same module again, it is not “run” again. Python keeps track of imported modules and just re-uses the already imported module the second time. This makes C/C++ tricks like enclosing header file bodies with IFNDEF statements to make sure the header is only imported once unnecessary in python.