[Tutorial & Examples] try block in Python

1) Motivation:

Why we need it?

open('demo_read.txt')

If the 'demo_read.txt' is not exist yet,  we will encounter 2 issues:

a) The script will exit immediately 

b) The script will print a bit verbose and messy error message:

Traceback (most recent call last):

  File "c:\Users\liuga\git_downloads\study\python\pickle_notes.py", line 1, in <module>

    open('demo_read.txt')

FileNotFoundError: [Errno 2] No such file or directory: 'demo_read.txt'

Can we continue run the script without exiting? 

Can we print out a clean or better customized error messages?

Yes, yes, Let's move on.

2) Better way!

try:

    open('demo_read.txt')

except FileNotFoundError as e:

    print(e)

The terminal will display:

[Errno 2] No such file or directory: 'demo_read.txt'

3) What is more?

Example 1:

try:

    open('demo_read.txt')

    x = 0

except FileNotFoundError as e:

    print(e)

except Exception as e:

    print(e)

else:

    print(f'Everthing is fine, no error!')

finally:

    print(f'The try-block is done no matter what happens. You will always see this line')

When running it, terminal will display:

Everthing is fine, no error!

The try-block is done no matter what happens. You will always see this line

Example 2:

try:

    open('demo_read.txt')

    x = y

except FileNotFoundError as e:

    print(e)

except Exception as e:

    print(e)

else:

    print(f'Everthing is fine, no error!')

finally:

    print(f'The try-block is done no matter what happens. You will always see this line')

When running it, terminal will display:

name 'y' is not defined

The try-block is done no matter what happens. You will always see this line

Example 3:

files = ['demo_read0.txt', 'demo_read.txt']

for file in files:    

    print(f'processing {file}')

    try:        

        f = open(file)                

    except FileNotFoundError as e:

        print(e)

    except Exception as e:

        print(e)

    else:

        print(f'Everthing is fine, no error for {file}')

        f.close()

    finally:

        print(f'The try-block is done no matter what happens. You will always see this line')

When running it, terminal will display:

processing demo_read0.txt

[Errno 2] No such file or directory: 'demo_read0.txt'

The try-block is done no matter what happens. You will always see this line

processing demo_read.txt

Everthing is fine, no error for demo_read.txt

The try-block is done no matter what happens. You will always see this line 

Conclusion:

Now I would like to sum up the try-block with my own language as this: it is a mechanism to handle error with more choices. 

This article is inspired by [1]

Reference:

[1] https://www.youtube.com/watch?v=NIWwJbo-9_8