Develop Reference

ModuleNotFoundError while importing from alias

e.g.
import os as my_os
import my_os.path
ModuleNotFoundError: No module named 'my_os'
but the following script is ok
import os
import os.path

You can't do that in Python.
import statements are importing from Python file names.
You aren't renaming the file of os to my_os, therefore this wouldn't work.
As mentioned in the documentation:
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.

Tried the similar way as os.py did:
import json as my_json
from my_json.decoder import * # ModuleNotFoundError: No module named 'my_json'
import sys
import json.decoder as my_decoder
sys.modules['my_json.decoder'] = my_decoder
from my_json.decoder import * # it's ok now

How to import a module from sub directory

I failed to import a module from sub directory in python. Below is my project structure.
./main.py
./sub
./sub/__init__.py
./sub/aname.py
when I run python main.py, I got this error:
Traceback (most recent call last):
File "main.py", line 4, in <module>
import sub.aname
File "/Users/dev/python/demo/sub/__init__.py", line 1, in <module>
from aname import print_func
ModuleNotFoundError: No module named 'aname'
I don't know it failed to load the module aname. Below is the source code:
main.py:
#!/usr/bin/python
import sub.aname
print_func('zz')
sub/__init__.py:
from aname import print_func
sub/aname.py:
def print_func( par ):
print ("Hello : ", par)
return
I am using python 3.6.0 on MacOS

There are several mistakes in your Python scripts.
Relative import
First, to do relative import, you must use a leading dots (see Guido's Decision about Relative Imports syntax).
In sub/__init__.py, replace:
from aname import print_func
with:
from .aname import print_func
Importing a function
To import a function from a given module you can use the from ... import ... statement.
In main.py, replace:
import sub.aname
with:
from sub import print_func

from sub import aname
aname.print_func('zz')

Probably the most elegant solution is to use relative imports in your submodule sub:
sub.__init__.py
from .aname import print_func
But you also need to import the print_func in main.py otherwise you'll get a NameError when you try to execute print_func:
main.py
from sub import print_func # or: from sub.aname import print_func
print_func('zz')

Module undefined after importing it in a function

I'm trying to use a function called start to set up my enviroment in python. The function imports os.
After I run the function and do the following
os.listdir(simdir+"main")
I get a error that says os not defined
code
>>> def setup ():
import os.path
import shutil
simdir="e:\\"
maindir="c:\\backup\\bitcois\\test exit\\"
>>> setup()
>>> os.listdir(simdir+"main")
Traceback (most recent call last):
File "<pyshell#10>", line 1, in <module>
os.listdir(simdir+"main")
NameError: name 'os' is not defined

The import statement is scoped. When importing modules they are defined for the local namespace.
From the documentation:
Import statements are executed in two steps: (1) find a module, and initialize it if necessary; (2) define a name or names in the local namespace (of the scope where the import statement occurs). [...]
So in your case the os package is only defined within function setup.

You are getting this error because you are NOT importing the whole os library but just the os.path module. In this way, the other resources at the os library are not made available for your use.
In order to be able to use the os.listdir method, you need to either import it alongside the os.path like this:
>>> def setup ():
import os.path, os.listdir
import shutil
simdir="e:\\"
maindir="c:\\backup\\bitcois\\test exit\\"
or import the full library:
>>> def setup ():
import os
import shutil
simdir="e:\\"
maindir="c:\\backup\\bitcois\\test exit\\"
You can read more here:
https://docs.python.org/2/tutorial/modules.html

try:
import os.path
import shutil
import glob
def setup ():
global simdir
simdir="e:\\"
maindir="c:\\backup\\bitcois\\test exit\\"
setup()
os.listdir(simdir+"main")

You need to return the paths and assign the returned values in the global scope. Also, import os too:
import os
def setup():
# retain existing code
return simdir, maindir
simdir, maindir = setup()

When you import os or do any sort of command within a function, the command's effect only last while that function itself is running. What you need to do is
import os
...Do your function and other code
This way, your import lasts for the whole program :).

Error in importing a python file

I've written a python file and am trying to import it but it's not recognized.
The file was saved as gentleboost_c_class.c in C:\User\apps\My documents.
I tried to import it like this:
import gentleboost_c_class as gbc
But I get this error:
NameError: name 'gentleboost_c_class' is not defined
gentleboost_c_class.py begins like this:
from sklearn.externals.six.moves import zip
import numpy as np
import statsmodels.api as sm
class GentleBoostC:
.....
It compiles fine.
Both files are in the same folder.
What am I doing wrong?

You're getting a NameError, not an ImportError.
So it seems to me that you import your module as gbc, but later try to refer to it as gentleboost_c_class.
If you import the module with
import gentleboost_c_class as gbc
that means it will be available under the global name gbc, but not as gentleboost_c_class.

ImportError when attempting to mock a module

I have a module that I am testing that depends on another module that won't be available at the time of testing. To get around this, I wrote (essentially):
import mock
import sys
sys.modules['parent_module.unavailable_module'] = mock.MagicMock()
import module_under_test
This works fine as long as module_under_test is doing one of the following import parent_module, import parent_module.unavailable_module. However, the following code generates a traceback:
>>> from parent_module import unavailable_module
ImportError: cannot import name unavailable_module
What's up with this? What can I do in my test code (without changing the import statement) to avoid this error?

Alright, I think I've figured it out. It appears that in the statement:
from parent_module import unavailable_module
Python looks for an attribute of parent_module called unavailable_module. Therefore, the following set up code fully replaces unavailable_module within parent_module:
import mock
import sys
fake_module = mock.MagicMock()
sys.modules['parent_module.unavailable_module'] = fake_module
setattr(parent_module, 'unavailable_module', fake_module)
I tested the four import idioms of which I am aware:
import parent_module
import parent_module.unavailable_module
import parent_module.unavailable_module as unavailabe_module
from parent_module import unavailable_module
and each worked with the above set up code.