Best Ways in Python to Import Classes From Another File

In this article, we will understand the need for modular programming and then will learn how to Import Classes from another file in the python programming language. We will discuss some examples to understand them more clearly. We will also understand how predefined libraries help us. So let’s get started.

Why is it needed?

So, the first question that sounds to our mind is that why we need to import any file or classes from another file? Or, Is it really important? Or, How it will affect our programming approach? Answers to all these questions are within this article.

First, understand why is it needed? In programming, we are often in situations in which we have to repeat certain operations. Writing code for them each and every time is quite a tedious task. To avoid such situations, we use the concept of Object-Oriented Programming aka OOPs.

Object Oriented Programming or OOPs

OOPs is a programming concept in which we create objects based on some defined classes having some properties (called attributes) and functions (called methods). The advantage of doing that is we can reuse it a number of times we want.

We can also implement other real-time entities like inheritance ( i.e. inheriting properties from other classes), abstraction ( i.e. information hiding), polymorphism (i.e. present in many forms) e.t.c. in it. It is an independent concept in itself, so we will talk about it somewhat later. For now, move ahead and talk about modular programming.

Modular Programming

Although, OOPs is an efficient and smart way of programming, we can’t say that it themselves solve all the problems alone. Besides the core concept of OOPs, there are some other problems we face while programming. Working on real scenarios is very different from the one that we learn.

In reality, solving any problem may require thousands of lines of codes which may have hundreds of functions and tens of classes. Maintaining all those functions and classes in one program is a difficult task. So we use the modular programming approach to cope with such situations.

Modular Programming is a programming approach in which rather than writing code for one big problem, we split it into several small independent problems. Once we solve each of them, we unite them to achieve our primary goal.

Advantages of using Modular Programming

The advantage of following the approach is that we can increase the readability of codes. We can reuse it a number of times. More, it is easy for us to maintain them and make changes to them according to our use. Moreover, it helps us a lot while debugging our program as we don’t have to fumble much in one file.

Introduction to Import Statement

Once all independent problems are solved, the question arises of how we can associate them. Here, the Python Import statement comes into the picture.

Import statement helps in importing code from other files or modules into the file we are working on. You can think of it as a bridge that gives us access to the class attributes or methods from other files so that we can use them. However, there is a proper syntax to follow to do that which we will see in a few seconds. Before that first understand the given directory structure.

application
  |
  |---- module1
  |       |------- __init__.py
  |       |------- file1.py
  |       |------- file2.py
  |
  |---- module2
          |------- file3.py

So, in the given directory structure we have an application folder that contains two folders named module1 and module2. Inside each folder, we have files named file1, file2, and file3 respectively. Now, we will discuss different scenarios of importing classes or files one by one. Here we go,

Import Classes From Another File

In this example, first, we will define a class in file1.py and then import it in file2.py.

application/module1/file1.py

class ArithmeticOperations:

    def __init__(self):
        self.sum_ = 0
    
    def get_sum(self,num1,num2):
        self.sum_ = num1+num2
        return self.sum_

application/module1/file2.py

import file1

obj = file1.ArithmeticOperations()
c = obj.get_sum(3,8)
print(c)

Output: 
11

In the above example, we imported file1 using the “import” statement. After that, we create an instance of the class we want to use i.e. ArithmeticOperations by referring to it. We can understand it as;

• We imported the file named file1.
• After that, we refer to that class using file1.ArithmeticOperations() and assign it to variable obj.
• After that, we call get_sum function by using ‘obj’ object.
• And, then print the result assigned in ‘c‘ variable.

We can also do it using “from <module_name> import <class_name> ” statement. Take a look at it;

from file1 import ArithmeticOperations

obj = ArithmeticOperations()
print(obj.get_sum(9,11))

Output:
20

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Import All classes from Another File

In this section, we will discuss how we can import all the classes at once from another file. Sometimes, it is the scenario that it is needed to import all the classes from other files. In that case, we use “from <file_name> import *” statement. We can read it as from file_name import all. It means that we imported all the available classes from the given file. Let’s look at its example.

application/module1/file1.py

class ArithmeticOperations:

    def __init__(self):
        self.sum_ = 0
    
    def get_sum(self,num1,num2):
        self.sum_ = num1+num2
        return self.sum_

class PrintData:

    def __init__(self):
        self.data = ""

    def print_data(self,data):
        self.data = data
        return self.data

application/module1/file2.py

from file1 import *

obj1 = ArithmeticOperations()
print(obj1.get_sum(9,11))

obj2 = PrintData()
print(obj2.print_data("Accessing Another class from file1"))

Output:
20
Accessing Another class from file1

Import Classes From Another Folder

In this example, we will use ArithmeticOperations class in file3.py i.e located in another folder i.e module1. But before doing that we have to include __init__.py file in module1 which tells the interpreter that the module is within the same package.

We also have to specify the path of module1. For that, we will use the “sys” module. Let’s look at the code.

import sys
sys.path.insert(0,"..")

from module1.file1 import ArithmeticOperations

obj = ArithmeticOperations()
print(obj.get_sum(8,23))

Output:
31

‘sys.path.insert(0,”..”) ‘ This line of code tells the interpreter that first come out of the current directory to parent directory and then look for the module there.

Import Classes From Predefined Libraries

In the above example, we used “import sys” which is a predefined library and we want to import it and use its functionalities.

sys.path.insert(); We can understand, that we want to use the insert method from the path class that is present in the sys module. Using such dependencies and modules make our works easier as we don’t need to write code for those function explicitly. We just need the knowledge of the given package and how to use it.

Python import class from Another File Absolute Path

Use the sys module to import from another file. Use the absolute path as the argument. 

Example:

import sys
sys.path.append('absolute path')

import module

Now after appending the filepath, you can import whichever class module you want.

Python import class from Another file Module Not Found

This can happen if the path which you have specified as argument can’t access the module. So to correct this error, check the path including the subdirectory or the directory that you have stated.

How to use __import__ to import a class from other files in Python?

We can use the following code in file2.py to import the class from file1 dynamically.

class Dynamic_import:
  def __init__(self,module_name,class_name):
    #__init__ method is used to import the module which takes module name as theparameter
    module = __import__(module_name)

    # getattr() is used to import class name form the module we imported
    class_nm = getattr(module,class_name)
    data= class_nm.get_sum(class_nm,3,8)
    print(data)

obj = Dynamic_import("file1","ArithmeticOperations")

How to import class dynamically?

We can import class dynamically using __import__ magic function or imp module.

Here’s an example –

import imp
import sys

#dynamic import
def dynamic_imp(name, class_name):
# imp.find_module() returns the path and description of given module
fp, path, desc = imp.find_module(name)

#imp.load_module() returns either class or package
package = imp.load_module(name, fp, path, desc)
class_ = imp.load_module("% s.% s" % (name, class_name), fp, path, desc)
return package, class

if name == "main":
mod, modCl = dynamic_imp("file1 ","addNumbers")
modCl.addNumbers(1, 2)

FAQs

Conclusion

In this article, we looked at how we can import classes from other files or folders. We also gained knowledge about Modular Programming and Import Statement.

I hope this article helps you. Thank You!