README.md

PythonTricks

This repo contains python practice programs that I worked out while studying 'Python Tricks' book by 'Dan Bader'.

Unit 2.1 - Assertions

• assert statements are great way to self-check if our program is working as expected.
• Each assert checks if an impossible condition happens.
• assert statements are not a mechanism for handling run-time errors.
• All assert statements can globally disabled by interpreter settings.
  • python -O program.py gets rid of all assertions in our program.
  • So assert statements are not meant for handling run-time errors as mentioned above.

Example(s)

• assert 10 == 9 - raises AssertionError
• assert (10 == 9) - does not raise AssertionError since first is evaluated as tuple and always returns True since it is non-empty.
• assert 10 == 9, '10 is Not Equal to 9' raises AssertionError with specified message

Unit 2.2 - Comma Placement in list, set or dict

• Span the list across multiple lines which will help us later to track which element added/removed/modified later using diff tools.
• It also helps the code reviewers to easily find the changes.
• Always add ',' to the end of every element including last element so that comma is not missed whenever a new element is added which will prevent from string literal concatentation problem (actually a feature).

Example(s)

names = [
   'John',
   'David',
   'Peter',
]

Unit 2.3 - Context Managers and the with Statement

• with statement makes code that deals with system resources more readable.
• It ensures that open file descriptors (or any system resource) are closed automatically after program execution leaves the context of with statement.

Examples(s)

f = open('hello.txt', 'w')
f.write('hello, world')
f.close()

# above can be replaced with below code which
# guarantees that the file is closed even if there's
# an exception during f.write(). More Verbose.
f = open('hello.txt', 'w')
try:
    f.write('hello, world')
finally:
    f.close()

# same as above but less verbose
with open('hello.txt', 'w') as f:
    f.write('hello, world!')

• How to provide the same functionality to our own classes and functions? Implementing Context-Managers

• What is context manager? Protocol (or interface) that our object or function needs to follow in order to support with statement.

Example(s)

# Using Decorators
from contextlib import contextmanager

@contextmanager
def managed_file(name):
    try:
        f = open(name, 'w') # acquire resource
        yield f # yields resource
    finally:
        f.close() # release resource

with managed_file('with_functions.txt') as f:
    f.write('hello, world!')
    f.write('bye now')

# By Implementing the interface of context-manager in class
class ManagedFile:
    def __init__(self, name):
        self.name = name
    
    # Acquires the resource when execution enters context.
    def __enter__(self):
        self.file = open(self.name, 'w')
        return self.file
    
    # Free up resource when execution leaves context.
    def __exit__(self, exc_type, exc_val, exc_tb):
        if self.file:
            self.file.close()

with ManagedFile('with_object_hello.txt') as f:
    f.write('hello, world!')
    f.write('bye now')

• Indenter exercise and timer exercise are available in indent_exercise.py and timer_exercise.py files respectivly under unit_2_3_with_statement directory.