OS Module and Exception Handling

os Module

What Is It?

This os module provides a portable way of using operating system dependent functionality.

How Does It Work?

The code below uses the os module to display the current working directory. This is equivalent to the console command ‘pwd’.

import os
cwd = os.getcwd()
cwd 
# '/Users/rd/'

Paths

A relative path relates to the location of a file relative to the current working directory.

An absolute path is a path that begins from the root directory of the operating system. Paths that start with a / (from the root directory) are not relative to the current directory.

import os
os.path.abspath('README.md')
# '/Users/rd/Dev/my_awesome_project/README.md'

Verify a file exists.

import os
os.path.exists('README.md')
# True

Verify a path points to a directory.

import os
os.path.isdir('/Users/rd')
# True

Verify a path points to a file.

import os
os.path.isfile('/Users/rd')
# False

List the contents of a directory.

import os
os.listdir('/Users/rd')
# ['Applications', 'Desktop', 'Documents', 'Downloads', '.bash_profile']

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Exceptions

Part 1

What Is It?

Exceptions are a means of breaking out of the normal flow of control of a code block in order to handle errors or other exceptional conditions.

fin = open('i_like_turtles.py', 'r')
# Traceback (most recent call last):
#   File "<stdin>", line 1, in <module>
# FileNotFoundError: [Errno 2] No such file or directory: 'i_like_turtles.py'

The Try Statement

The try statement specifies exception handlers and/or cleanup code for a group of statements.

try:
    fin = open('i_like_turtles.py', 'r')
except FileNotFoundError:
    print('The file does not exist!')

# The file does not exist!

The except Clause

The except clause(s) specifies one or more exception handlers. When no exception occurs in the try clause, no exception handler is executed.

try:
    print(100 / 10)
except ZeroDivisionError:
    print("Division by zero is not allowed.")

# 10.0

The else Clause

A way to specify a block of code should be run only if no exceptions were raised at all.

try:
    print(100 / 10)
except ZeroDivisionError:
    print("Division by zero is not allowed.")
else:
    print("No issues were encountered.")

# 10.0
# No issues were encountered.

try:
    print(100 / 0)
except ZeroDivisionError:
    print("Division by zero is not allowed.")
else:
    print("No issues were encountered.")

# Division by zero is not allowed.

The finally clause

If finally is present, it specifies a ‘cleanup’ handler.

try:
    print(100 / 10)
except ZeroDivisionError:
    print("Division by zero is not allowed.")
finally:
    print("This runs no matter what happened.")

# 10.0
# This runs no matter what happened.

try:
    print(100 / 0)
except ZeroDivisionError:
    print("Division by zero is not allowed.")
finally:
    print("This runs no matter what happened.")

# Division by zero is not allowed.
# This runs no matter what happened.

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Exceptions

Part 2

Catching Multiple Exceptions

There can be multiple exception handlers.
If an exception occurs, the handler searches for a a clause that matches the exception.
If an expression-less except clause is present, it must be declared last.

try:
    ...
except FileNotFoundError:
    ...
except PermissionError:
    ...
except:
    # No other exceptions matched

Catching Multiple Exceptions With A Base Class

If the logic to handle two or more exceptions is the same, a base class can be used.

try:
    ...
except OSError:  # matches FileNotFoundError, PermissionError, and others... 
    ...

Catching Multiple Exceptions With A Tuple

If the logic to handle two or more exceptions is the same, a tuple can be used.

try:
    ...
except (FileNotFoundError, PermissionError):
    ...

Accessing The Exception Object

import logging

try:
    ...
except Exception1 as e:
    logging.error(e)
except (Exception2, Exception3) as e:
    logging.error(e)

raise Keyword

if(condition):
    raise Exception('Something went wrong')

Exception Chains

try:
    ...
except Exception as e:
    raise Exception2('Something went wrong')

Built-in Exceptions

Built-in Exceptions Docs

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Persistence

Files

The open Built-in Function

Use the open built-in function to acquire a file object.

fout = open('new_file', 'w')
fout.write("Hello file\n")
# 11
fout.close()
fin = open('new_file', 'r')
for line in fin:
    print(line)

# Hello file
fout.close()

Available Modes

character	Meaning
r	open for reading (default)
w	open for writing, truncating the file first
x	open for exclusive creation, failing if the file already exists
a	open for writing, appending to the end of the file if it exists
b	binary mode
t	text mode (default)
+	open a disk file for updating (reading and writing)

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The with Statement

The with statement is used to wrap the execution of a block with methods defined by a context manager.

The context manager will automatically close the file handler when you are done with it.

Opening Files Using With Statement

with open('new_file', 'r') as file_handler:
    for line in file_handler:
        print(line)

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The End

Parrot