Python Tutorial - 8 - Import Modules and Exploring the Standard Library

Import Modules and Exploring the Standard Library

/*

• File: import modules-standard library.md
• Project: 8_import
• File Created: Thursday, 16th March 2023 10:18:07 pm
• Author: Hanlin Gu ([email protected])

---

• Last Modified: Saturday, 18th March 2023 4:27:33 pm
• Modified By: Hanlin Gu ([email protected]>)
  */

1. Import

1.1. my_module.py

print('Imported my_module...')

test = 'Test String'


def find_index(to_search, target):
    """Find the index of a target value in a sequence"""
    for i, value in enumerate(to_search):
        if value == target:
            return i

    return -1

1.2. import module_name

Put the module in the same folder as the current python file. And use import module_name to import the module_name.py file.

import my_module

courses = ['History', 'Math', 'Physics', 'CompSci']

index = my_module.find_index(courses, 'Math')

print(index)

Output:

Imported my_module...
1

1.3. Shorter module name

To make the module name shorter, one can define a shorter name by import module_name as module_shorter. When call the module, one can use module_shorter instead.

import my_module as mm

courses = ['History', 'Math', 'Physics', 'CompSci']

index = mm.find_index(courses, 'Math')

print(index)

Output:

Imported my_module...
1

1.4. Import the Function Itself

from my_module import find_index

courses = ['History', 'Math', 'Physics', 'CompSci']

index = find_index(courses, 'Math')

print(index)

Output:

Imported my_module...
1

One thing to note about this approach, it only gives access to the find_index() function not everything else in the module.

as still works to make a shortcut for a function by from module_name import func_name as func_shorter. But this will reduce the readability of the code, so it’s not recommended to do very often.

from my_module import find_index as fi, test

courses = ['History', 'Math', 'Physics', 'CompSci']

index = fi(courses, 'Math')

print(index)
print(test)

Output:

Imported my_module...
1
Test String

To import every thing, one can use from module_name import *. This is not used very often, because it’s hard to track the original module of a function.

1.5. Locations that import searches

When import a module or the standard library, the system is gonna check a multiple locations. And the locations that are checked, is within a list called sys.path.

import sys
print(sys.path)

Output:

['d:\\Documents\\Code\\python\\tutorial\\9_import modules-library', 'D:\\Program Files\\Python\\Python310\\python310.zip', 'D:\\Program Files\\Python\\Python310\\DLLs', 'D:\\Program Files\\Python\\Python310\\lib', 'D:\\Program Files\\Python\\Python310', 'D:\\Program Files\\Python\\Python310\\Lib\\site-packages', 'D:\\Program Files\\Python\\Python310\\Lib\\site-packages\\win32', 'D:\\Program Files\\Python\\Python310\\Lib\\site-packages\\win32\\lib', 'D:\\Program Files\\Python\\Python310\\Lib\\site-packages\\Pythonwin']

It contains the current directory of the script that is running, Python path environment variable, standard library directory, site-packages directory for 3rd party packages.

If the imported module is not in the folder, there’s a ModuleNotFoundError appears.

from my_module import find_index, test

courses = ['History', 'Math', 'Physics', 'CompSci']

index = find_index(courses, 'Math')

print(index)
print(test)

Output:

Traceback (most recent call last):
  File "d:\Documents\Code\python\tutorial\9_import modules-library\9_import modules-standard library.py", line 40, in 
    from my_module import find_index, test
ModuleNotFoundError: No module named 'my_module'

1.6. Add Python Path

1.6.1. Direct Method: sys.path.append('folder_address')

# Append Path
import sys
sys.path.append('D:\Documents\Code\python\HG_modules')

from my_module import find_index

courses = ['History', 'Math', 'Physics', 'CompSci']
index = find_index(courses, 'Math')

print("The index of 'Math' in courses is {}".format(index))
# print(f"The index of 'Math' in courses is {index}")
print(sys.path)

Output:

Imported my_module...
The index of 'Math' in courses is 1
['d:\\Documents\\Code\\python\\tutorial\\9_import modules-library', 'D:\\Program Files\\Python\\Python310\\python310.zip', 'D:\\Program Files\\Python\\Python310\\DLLs', 'D:\\Program Files\\Python\\Python310\\lib', 'D:\\Program Files\\Python\\Python310', 'D:\\Program Files\\Python\\Python310\\Lib\\site-packages', 'D:\\Program Files\\Python\\Python310\\Lib\\site-packages\\win32', 'D:\\Program Files\\Python\\Python310\\Lib\\site-packages\\win32\\lib', 'D:\\Program Files\\Python\\Python310\\Lib\\site-packages\\Pythonwin', 'D:\\Documents\\Code\\python\\HG_modules']

1.6.2. Add the Folder to Python Environment

The above is not the best way to add the address to python, because it append before other import. It is better to change the environment variable, and doesn’t need to add the sys.path.append() at each time modules from certain location is called.

Mac

In the terminal, use nano to change the .bash_profile file. ~/ means working in the home directory.

nano ~/.dash_profile

Scroll to the end of the file and set the python path.

export PYTHONPATH="/Users/your_user_name/file_location"

Here, your_user_name is your logged in user name, and file_location is where the module is located. Note that, no space in between the equal and the path.

Hit ctrl + x, then y to save the change, and enter to keep the same file name. Finally, to restart the terminal.

Windows

Right click Computer 此电脑 $\to$ Properties 属性 $\to$ Advanced system settings 高级系统设置 $\to$ Environment Variable 环境变量

• In the User variables 用户变量, click New 新建, set Variable name 变量名 as PYTHONPATH, and Variable value as the folder address of the self-defined modules. Finally click ok to save the change.

import sys
print(sys.path)

Output:

['d:\\Documents\\Code\\python\\tutorial\\9_import modules-library', 'D:\\Documents\\Code\\python\\HG_modules', 'D:\\Program Files\\Python\\Python310\\python310.zip', 'D:\\Program Files\\Python\\Python310\\DLLs', 'D:\\Program Files\\Python\\Python310\\lib', 'D:\\Program Files\\Python\\Python310', 'D:\\Program Files\\Python\\Python310\\Lib\\site-packages', 'D:\\Program Files\\Python\\Python310\\Lib\\site-packages\\win32', 'D:\\Program Files\\Python\\Python310\\Lib\\site-packages\\win32\\lib', 'D:\\Program Files\\Python\\Python310\\Lib\\site-packages\\Pythonwin']

The 1st address is the current working directory. It’s clear that the 2nd folder is added to the python environment successfully.

2. Exploring the Standard Library

2.1. random module

• The random.choice() picks up a random element in a sequence.

import random

courses = ['History', 'Math', 'Physics', 'CompSci']

random_course = random.choice(courses)

print(random_course)

Output:

Math

2.2. math module

math is for common mathematical operations.

• Convert 90 degree to radians:

import math

rads = math.radians(90)

print(rads)
print(math.sin(rads))

Output:

1.5707963267948966
1.0

2.3. datetime and calendar modules

import datetime
import calendar

today = datetime.date.today()
print(today)

print(calendar.isleap(2023))

Output:

2023-03-17
False

2.4. os module

os gives access to the underlying operating system.

• print current working directory

import os

print(os.getcwd())

Output:

d:\Documents\Code\python\tutorial\9_import modules-library

os module has tons of other functionalities. It gives the ability to scan the file system, and create files, delete files.

2.4.1. Find the Location of a Module

The standard library modules are simply python files. To view the location of a module, just print out its os.__file__.

import os
import numpy as np

print(os.__file__)
print(np.__file__)

Output:

D:\Program Files\Python\Python310\lib\os.py
D:\Program Files\Python\Python310\Lib\site-packages\numpy\__init__.py

And D:\Program Files\Python\Python310\lib is the standard library directory and it’s all the standard modules that are located.