reading-notes

401 Class-03 FileO and Exceptions

Reading and Writing Files in Python

• files are a set of bytes used to store data

• bytes are translated into binary

• Files contain three main parts
  • Header - metadata contents of file
  • Data - contents of file written by creator/editor
  • EOF - End of File, special char indicated end of file
• File Paths
  • Folder Path - file folder location
  • File name - name of file
  • iExtension - end of file path pre-pended with period. indicates file type
• Line endings - new line, around since morse code
• Character Encoding - translation from bye data to human readable, ASCII or UNICODE

• Opening and Closing a file

  file = open('file_name.txt')
  

  reader = open('file_name.txt')
  try:
  finally:
    reader:close
  

    with open('file_name.txt') as reader
  

  • mode

    with python('file_name.txt', 'r') as reader:
  

• Three different file objects
  • Text files
  • Buffered binary files
  • Raw binary files
• Reading and Writing opened files
  • .read(size=-1)
  • .readline(size=-1)
  • .readlines()
• Iterate over each line

>>> with open('dog_breeds.txt', 'r') as reader:
>>>     # Read and print the entire file line by line
>>>     line = reader.readline()
>>>     while line != '':  # The EOF char is an empty string
>>>         print(line, end='')
>>>         line = reader.readline(

• .write(string)
• .writelines(seq)

with open('dog_breeds.txt', 'r') as reader:
    # Note: readlines doesn't trim the line endings
    dog_breeds = reader.readlines()

with open('dog_breeds_reversed.txt', 'w') as writer:
    # Alternatively you could use
    # writer.writelines(reversed(dog_breeds))

    # Write the dog breeds to the file in reversed order
    for breed in reversed(dog_breeds):
        writer.write(breed)

• byte strings - add b character to mode

Python Exceptions

• Syntax errors - parser detects incorrect statement
• Exception errors - syntactically correct Python code results in error
  • raise - used to throw an exception if condition occurs
• AssertionError Exception - assert that a certain condition is met
• try and except - catch and handle exception
• Else - execute a certain block of code in absence of exceptions
• Finally - will always run

Home