Importing entire script instead of function

[member 428835]

Hello

I have a script a13_roman2num.py that reads

def value(r):

    if (r == 'I'):
        return 1
    if (r == 'V'):
        return 5
    if (r == 'X'):
        return 10
    if (r == 'L'):
        return 50
    if (r == 'C'):
        return 100
    if (r == 'D'):
        return 500
    if (r == 'M'):
        return 1000
    return

def romanToDecimal(str):
    i   = 0;
    res = 0;

    while i < len(str):
        if i + 1 < len(str):
            if value(str[i]) < value(str[i+1]):
                res += value(str[i+1]) - value(str[i])
                i += 2
            else:
                res += value(str[i])
                i += 1
        else:
            res += value(str[i])
            i += 1

    return res

# DRIVER CODE
str = "MCMIV"
print(romanToDecimal(str))

and another script a12.py that runs

from a13_roman2num import value

For some reason when I run a12.py the output prints 1904. Why is a12.py running the entire script a13_roman2num.py instead of just importing the function I'm asking for?

Thanks so much!

 

[phyzguy]

This is a good question. I confirm the same behavior you see. I would not have expected it to do that. If you put the 3 statements at the end inside a function, like main() for example, then it doesn't do that. I guess it has to do with the details of how the import function works. This link gives some details, but doesn't really answer your question.
https://docs.python.org/3/reference/import.html

 

[jedishrfu]

Isn’t that why programmers use the

    if __name__ == ‘__main__’:
        print(…)

coding scheme to prevent the code outside all functions in a13 from runnning?

https://www.freecodecamp.org/news/if-name-main-python-example/

 

[member 428835]

Isn’t that why programmers use the

    if __name__ == ‘__main__’:
        print(…)

coding scheme to prevent the code outside all functions in a13 from runnning?

https://www.freecodecamp.org/news/if-name-main-python-example/

Okay, this makes sense. But does this mean every time I write a script I need to have

    if __name__ == ‘__main__’:
        print(…)

preceding all executions to ensure if I'm calling on a function from that script I don't import the executions as well?

 

[phyzguy]

Maybe the answer is that it needs to run the code outside of the functions in order to bring in any imports you have. For example, if you have:

import numpy as np

def Sqrt(x):
    return np.sqrt(x)

And you then want to import the function Sqrt() into another python script, you would expect it to run the import statement or else the Sqrt() function won't work. So it may be set up to run anything outside of def statements when it does the import.

 

[Mark44]

For some reason when I run a12.py the output prints 1904. Why is a12.py running the entire script a13_roman2num.py instead of just importing the function I'm asking for?

What's the full text of a12.py? All you showed was just the import statement.

 

[PeterDonis]

Why is a12.py running the entire script a13_roman2num.py instead of just importing the function I'm asking for?

Because when you import a module, which is what your Python file that includes the value function is, all of the code in that module gets executed. Note that this includes function definitions: function definitions are executed at runtime in Python. So even if the only code your module included was function definitions, all of them would get executed when you imported the module, not just the value function that you want to use.

However, your module does not just include function definitions. It also includes statements. Those statements will get executed every time you import the module. If you don't want that, you need to use the if __name__ == '__main__' idiom that @jedishrfu referred to.

 

[PeterDonis]

Maybe the answer is that it needs to run the code outside of the functions in order to bring in any imports you have.

The answer is that when you import a module in Python, all of the code in that module runs, as I said in post #7. That is how Python works.

 

[PeterDonis]

What's the full text of a12.py? All you showed was just the import statement.

Wouldn't matter. The import statement alone is sufficient to produce the observed behavior.

 

[PeterDonis]

But does this mean every time I write a script I need to have

    if __name__ == ‘__main__’:
        print(…)

preceding all executions to ensure if I'm calling on a function from that script I don't import the executions as well?

It is a good general practice to not have any top-level code (i.e., code not inside a function or class definition) in a module (and every script is a module), other than import statements. So yes, it is good practice to put any top-level code inside an if __name__ == '__main__' block. There are cases where you do need to have top-level code inside a module and not in such a block, but they are rare.

 

[jedishrfu]

We often use the trick to add test code to a script that is only run when the script is invoked directly. When imported the test code is skipped.

 

[Ibix]

Just to reiterate what others have said, imagine a module

# Module foo.py
def x():
    return y()
def y()
    return "y"

If the python developers had tried to implement from foo import x as only compiling the function x then a call to x() wouldn't work because it wouldn't have heard of y.

 

[FactChecker]

Or if your a13_roman2num.py code enclosed the driver code in a function definition, then importing it would only define the function, rather than executing the driver code.

 

[Mark44]

Here's what works for me.

I changed the name of a13_roman2num.py to mccraney_module.py. I also removed the driver code from it.

I placed the driver code in a script named mccraney_main.py:

from mccraney_module import romanToDecimal

str = "MCMIV"
print(romanToDecimal(str))

Output is 1904

If I add this line to code immediately above...

print(value(str[0]))

...
I get "NameError: name "value" is not defined."
This shows that only the label romanToDecimal is being imported, which is what you would expect from the import statement.

 

[member 428835]

Thank you all for the detailed explanations. I have a much better idea what is going on.

 

[PeterDonis]

This shows that only the label romanToDecimal is being imported

Only that name is imported into the namespace of the module "mcraney_main.py". But note that all of the function definitions in "a13_roman2num.py" are still executed by the interpreter and are still in the namespace of the module "a13_roman2num.py" (as you can verify by looking up that module in sys.modules and seeing what attributes it has).

 

[PeterDonis]

Here's what works for me.

The same thing could be accomplished, as has already been mentioned, by putting the "driver code" in "a13_roman2num.py" in an if __name__ == '__main__' block. Which solution is preferred would depend on the specific use case: do you want to be able to run "a13_roman2num.py" itself as a script, or is it OK to have the "driver code" in a separate script and have "a13_roman2num.py" only be useful as an imported module?

 

[Mark44]

The same thing could be accomplished, as has already been mentioned, by putting the "driver code" in "a13_roman2num.py" in an if __name__ == '__main__' block. Which solution is preferred would depend on the specific use case: do you want to be able to run "a13_roman2num.py" itself as a script, or is it OK to have the "driver code" in a separate script and have "a13_roman2num.py" only be useful as an imported module?

My preference would be to separate out the functions and possibly initialization code into modules that are imported into other scripts, but not run on their own.

 

[PeterDonis]

My preference would be to separate out the functions and possibly initialization code into modules that are imported into other scripts, but not run on their own.

Yes, and that's a common pattern. But there are use cases where it makes sense to have a module be both runnable as a script and importable by other modules. (The Python interpreter has the -m switch to make it easy to run modules as scripts for this reason.)