【toturial】import function of python

There are many ways to import modules or packages.

1. regular imports
2. from __ import __
3. relative imports
4. optional imports
5. local imports

regular imports

import single module or package

import sys

import sys as system

import multi-modules or packages
import os, sys, time

import sub module or package
import urllib.error

from __ import __

from is used when you only want to import one part of a module and package

from functools import lru_cache
from os import path, walk, unlink
from os import uname, remove

from os import (path, walk, unlink, uname, 
                remove, rename)
from os import path, walk, unlink, uname, \
                remove, rename

from is used when you want to import all of a module and package

from os import *

relative imports

Case 1: 

Given a file structure like this

we want to import subpackage 1 and subpackage 2 in the top __init__.py

from . import subpackage1

from . import subpackage2

But it will occur a problem like 

The reason is that this subpackage is not initialized. 

The solution to this problem is to add two commands before using "from . import subpackage1"

import subpackage1

import subpackage2

from . import subpackage1

from . import subpackage2

Then in both subpackage1 and subpackage2, there generates a file directory named __pycache__ for each other. 

Once generating this file, next time you can directly use 

from . import subpackage1

from . import subpackage2

You may notice the file in the directory __pycache__ is __init__.... 

what does that mean?

let's go on and look back later

If we want to import module_x and module_y in __init__.py under subpackage1 

from . import module_x

from . import module_y

the same problem will occur

Then we use the method described above, as

import module_x

import module_y

from . import module_x

from . import module_y

Then you will find

__init__.cpython-37.pyc

module_x.cpython-37.pyc

module_y.cpython-37.pyc

So that looks like a registration, once appearing in this directory, then you can use them.

Case 2: 

If we want to use my_package as well as its packages and modules inside.

For example, we create a testpackage, test.py

So the way of import my_package is:

import sys

sys.path.append('C:/Users/acw393/Dropbox/SecondYearResearch')

import my_package

from my_package import module_a

Attention, the path should be the upper directory of  my_package

This case is very useful when you use local modules!!

Here is an example to use local modules! (attention: this case is that target module and current runfile are not in the same file directory)

the location of the target module

__init__.py is the initial function of the whole module. if you import seglearn, the __init__ will automatically run.

But I want to use this module named seglearn in a file,

What we should do is to:
1) add a sys path of current target module into system environment:
__init__.py :
import sys
sys.path.append('C:/Users/Bang/Dropbox/SecondYearResearch/Seglearn-revised')
import seglearn

2) add the same thing at any runfile where you directly use seglearn.
import sys
sys.path.append('C:/Users/Bang/Dropbox/SecondYearResearch/Seglearn-revised')
import seglearn

Here, we can know, if we create a module (which should have __init__.py to initially run all sub-modules), we only need to add following code in the  __init__.py of that module.
import sys
sys.path.append(path): path means the file directory where the defined module is. 
import name_definedmodule

then add the same code in the file you want to import.

Optional imports

It is used when you wish to use a certain module by priority or use a backup when the target module doesn't exist. 

try:
    # For Python 3
    from http.client import responses
except ImportError:  # For Python 2.5-2.7
    try:
        from httplib import responses  # NOQA
    except ImportError:  # For Python 2.4
        from BaseHTTPServer import BaseHTTPRequestHandler as _BHRH
        responses = dict([(k, v[0]) for k, v in _BHRH.responses.items()])

try:
    from urlparse import urljoin
    from urllib2 import urlopen
except ImportError:
    # Python 3
    from urllib.parse import urljoin
    from urllib.request import urlopen

Local imports

import sys  # global scope

def square_root(a):
    # This import is into the square_root functions local scope
    import math
    return math.sqrt(a)

def my_pow(base_num, power):
    return math.pow(base_num, power)

if __name__ == '__main__':
    print(square_root(49))
    print(my_pow(2, 3))

circular imports

# a.py
import b

def a_test():
    print("in a_test")
    b.b_test()

a_test()

import a

def b_test():
    print('In test_b"')
    a.a_test()

b_test()

Shadowed imports

import math

def square_root(number):
    return math.sqrt(number)

square_root(72)

Tips for establishing your own module and package

1. import all you need in __init__.py. This is a good habit for a programmer.

Reference

http://codingpy.com/article/python-import-101/