This is my module:
test1.py
regions=["a","b","c"]
print "from test1 module"
test2.py
from test1 import regions
print "from test2 module", regions
Running the test2.py
python test2.py
from test1 module
from test2 module ['a', 'b', 'c']
I see that the print
statement from test1.py
is called, although I only import regions
list from test1.py
. I don not say import test1.py
for everything to be executed.
1) why does it execute everything in test1.py
file (of course not the __name__==__main__
if included).
2) How to just import
the regions list
from test1 module
without executing all the other statements?
I did not know this is how import works and I have been working on a bug due to this for 3 days.
This happens because when Python imports a module, it runs all the code in that module. After running the module it takes whatever variables were defined in that module, and it puts them on the module object, which in our case is salutations .
When a module is first imported, Python searches for the module and if found, it creates a module object 1, initializing it. If the named module cannot be found, a ModuleNotFoundError is raised. Python implements various strategies to search for the named module when the import machinery is invoked.
The entire file becomes a module and can be imported inside another file to refer to the code. With module functionality, you can break your code into different files instead of writing everything inside one file. Later, using import, you can refer to the code inside the file you need.
The import statement reads the code in a Python module and allows you to use it in another file. Many of the modules you can use in your programs are part of the Python Standard Library.
That is just how imports work.
def my_function():
print("Hello")
What is the above snippet of code? It is a function definition for sure, but function definitions in Python are statements, and they must be executed in order to define the function. So when you import the above module, it executes the def
, which creates a new function and assigns it to my_function
. It's basically the same as:
my_function = ...definition..
So when you import a module, you always execute its contents. Otherwise you wouldn't be able to use any functions (or classes) in that module.
There are other ways to define functions in Python for sure.
def create_my_function(x):
def local_function():
print(x)
global my_function
my_function = local_function
create_my_function("Hello")
This is broadly equivalent to the original definition of my_function()
.
Since you can put any statements in a Python module, Python has no way of knowing which statements must be executed in order to define the symbols you are interested in. So it must execute all the statements.
Python needs to execute all statements in a module because they may have related side effects. For example consider
a = 2
b = a*3
What happens if I import b from that module? In general, modules other than main should not have side effects that are harmful when imported. In general modules should define classes, functions and variables, but should not do things like open files, connect to databases etc.
If you love us? You can donate to us via Paypal or buy me a coffee so we can maintain and grow! Thank you!
Donate Us With