Python tutorials: files, exception handling, and more

Pan Chuang AI 2020-11-13 12:49:03
python tutorials files exception handling

author |Vishal Mishra compile |VK source |Towards Data Science

Welcome to Python course . In this chapter , We are going to study the document 、 Exception handling and other concepts . Let's start .

__name__ == '__main__' What does that mean? ?

Usually , At every Python In the project , We'll all see the above statement . So what is it for , We are here to understand .

In short , stay Python in ,__name__ It's a special variable , It tells us the name of the module . Whenever it runs directly python file , It sets special variables before executing the actual code .__name__ It's a special variable . Determine according to the following points __name__ The value of the variable -

  1. If it runs directly python file ,__name__ The name will be set to main.

  2. If you import a module into another file ,__name__ The name will be set to the module name .

'__main__' Direct operation Import from other modules


In, Running from Import
In Second module’s name: main

In the example above , You can see , When you're in another python When the first module is imported into the file , It will enter else Conditions , Because the name of the module is not main. however , stay, The name is still main.

So we use it under the following conditions

  1. When we want to perform certain specific tasks , We can call this file directly .

  2. If the module is imported into another module , And when we don't want to perform certain tasks .

It's best to create a main Method , And in if __name__ == __main__ Internal calls . therefore , if necessary , You can still call from another module main Method .

We can still call main Method to call another module's main Method , because main Methods should exist in the first module .

What to do if something goes wrong

Python Exception handling in

When we write any program in any programming language , Sometimes even if the statement or expression is syntactically correct , There are also errors in the execution process . Errors detected during the execution of any program are called exceptions .

Python The basic terminology and syntax used to handle errors in is try and except sentence . Code that can cause an exception to occur is placed in try In block , Exception handling in except Implementation in block .python The syntax for handling exceptions in is as follows -

try and except

Do your operation …
except ExceptionI:
If there is any abnormality ExceptionI, Execute this block .
except ExceptionII:
If there is any abnormality ExceptionII, Execute this block .
If there is no abnormality , Then execute this block .
No matter whether there is any abnormality , This block will always execute 

Let's use an example to understand this . In the following example , I'm going to create a function to square numbers , In order to calculate the square , The function should always accept a number ( In this case, it's an integer ). But the user doesn't know about him / What kind of input does she need to provide . When the user enters a number , It works well , But if the user provides a string instead of a number , What's going to happen .

def acceptInput():
num = int(input("Please enter an integer: "))
print("Sqaure of the the number {} is {}".format(num, num*num))
Please enter an integer: 5
Sqaure of the the number 5 is 25

It throws an exception , The program suddenly ended . therefore , To execute programs gracefully , We need to handle exceptions . Let's look at the following example -

def acceptInput():
num = int(input("Please enter an integer: "))
except ValueError:
print("Looks like you did not enter an integer!")
num = int(input("Try again-Please enter an integer: "))
print("Finally, I executed!")
print("Sqaure of the the number {} is {}".format(num, num*num))
Please enter an integer: five
Looks like you did not enter an integer!
Try again-Please enter an integer: 4
Finally, I executed!
Sqaure of the the number 4 is 16

such , And we can provide logic for handling exceptions . But in the same example , If the user enters a string value again . What will happen then ?

So in this case , It's best to enter... In a loop , Until the user enters a number .

def acceptInput():
while True:
num = int(input("Please enter an integer: "))
except ValueError:
print("Looks like you did not enter an integer!")
print(" enterted integer finally so breaking out of the loop")
print("Sqaure of the the number {} is {}".format(num, num*num))
Please enter an integer: six
Looks like you did not enter an integer!
Please enter an integer: five
Looks like you did not enter an integer!
Please enter an integer: four
Looks like you did not enter an integer!
Please enter an integer: 7 enterted integer finally so breaking out of the loop
Sqaure of the the number 7 is 49

How to handle multiple exceptions

Can be in the same try except Block to handle multiple exceptions . You can have two ways -

  1. Provide different exceptions on the same line . Example :ZeroDivisionError,NameError :

  2. Provides multiple exception blocks . When you want to provide a separate exception message for each exception , It's very useful . Example :

except ZeroDivisionError as e:
print(“Divide by zero exception occurred!, e)
except NameError as e:
print(“NameError occurred!, e)

Include at the end except Exception:block It's always good , You can catch any unwanted exceptions you don't know about . This is a general exception capture command , It will cause any type of exception in the code .

# Handle multiple exceptions
def calcdiv():
x = input("Enter first number: ")
y = input("Enter second number: ")
result = int(x) / int(y)
print("Result: ", result)
except ZeroDivisionError as e:
print("Divide by zero exception occured! Try Again!", e)
except ValueError as e:
print("Invalid values provided! Try Again!", e)
except Exception as e:
print("Something went wrong! Try Again!", e)
print("Program ended.")
Enter first number: 5
Enter second number: 0
Divide by zero exception occured! Try Again! division by zero
Program ended.

How to create a custom exception

It's possible to create your own exceptions . You can use it. raise Keywords to do .

The best way to create a custom exception is to create a class that inherits the default exception class .

This is it. Python Exception handling in . You can see a complete list of built-in exceptions here :

How to deal with documents

Python File processing in

Python Use file objects to interact with external files on your computer . These file objects can be any file format on your computer , It can be an audio file 、 Images 、 text file 、 E-mail 、Excel file . You may need different libraries to handle different file formats .

Let's use ipython Command to create a simple text file , We'll learn how to do it in Python Read from the file .

%%writefile demo_text_file.txt
hello world
i love ipython
jupyter notebook
fourth line
fifth line
six line
This is the last line in the file
Writing demo_text_file.txt

Open file

You can open a file in two ways

  1. Define an inclusion file Object's variables . After processing a file , We have to use file Object methods close Turn it off again :

    f = open("demo_text_file.txt", "r")
  2. Use with keyword . There is no need to explicitly close the file .

    with open(“demo_text_file.txt”, “r”):
    ## Read the file 

stay open In the method , We have to pass the second parameter that defines the file access pattern .“r” It's for reading files . Similarly ,“w” Means write ,“a” Indicates that it is attached to a file . In the table below , You can see more common file access patterns .

Read the file

stay python in , There are many ways to read a file -

  1.> The entire file will be read into the string .

  2. fileObj.readline() => The file will be read line by line .

  3. fileObj.readlines()=> The entire file will be read and a list will be returned . Use this method with care , Because this will read the entire file , So the file size should not be too large .

# Read entire file
print("------- reading entire file --------")
with open("demo_text_file.txt", "r") as f:
# Read the file line by line
print("------- reading file line by line --------")
print("printing only first 2 lines")
with open("demo_text_file.txt", "r") as f:
# Read the file and return it as a list
print("------- reading entire file as a list --------")
with open("demo_text_file.txt", "r") as f:
# Use for Loop read file
print("\n------- reading file with a for loop --------")
with open("demo_text_file.txt", "r") as f:
for lines in f:
------- reading entire file --------
hello world
i love ipython
jupyter notebook
fourth line
fifth line
six line
This is the last line in the file
------- reading file line by line --------
printing only first 2 lines
hello world
i love ipython
------- reading entire file as a list --------
['hello world\n', 'i love ipython\n', 'jupyter notebook\n', 'fourth line\n', 'fifth line\n', 'six line\n', 'This is the last line in the file\n']
------- reading file with a for loop --------
hello world
i love ipython
jupyter notebook
fourth line
fifth line
six line
This is the last line in the file

Writing documents

And read similar ,python The following are provided 2 A way to write a file .

  1. fileObj.write()

  2. fileObj.writelines()

with open("demo_text_file.txt","r") as f_in:
with open("demo_text_file_copy.txt", "w") as f_out:

Read write binary

You can use binary mode to read and write any image file . Binary contains data in byte format , This is the recommended way to process images . Remember to use binary mode , With “rb” or “wb” Mode open file .

with open("cat.jpg","rb") as f_in:
with open("cat_copy.jpg", "wb") as f_out:
print("File copied...")
File copied...

Sometimes when the file is too large , Block reading is recommended ( Read fixed bytes at a time ), In this way, there will be no out of memory exceptions . You can provide any value for the block size . In the following example , You'll see how to read a file in a block and write to another file .

### Copy the image with a block
with open("cat.jpg", "rb") as img_in:
with open("cat_copy_2.jpg", "wb") as img_out:
chunk_size = 4096
img_chunk =
while len(img_chunk) > 0:
img_chunk =
print("File copied with chunks")
File copied with chunks


Now you know how to do exception handling and how to use Python Documents in .

Here is Jupyter Notebook Link to :

Link to the original text :

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