I. File Handling

There are four different methods (modes) for opening a file:

"r" - Read - Default value. Opens a file for reading, error if the file does not exist
"a" - Append - Opens a file for appending, creates the file if it does not exist
"w" - Write - Opens a file for writing, creates the file if it does not exist
"x" - Create - Creates the specified file, returns an error if the file exists

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In addition you can specify if the file should be handled as binary or text mode

"t" - Text - Default value. Text mode

"b" - Binary - Binary mode (e.g. images)

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Syntax

f = open("demofile.txt") # Make sure the file exists, or else you will get an error.

# is the same as:
f = open("demofile.txt", "rt")

# Because "r" for read, and "t" for text are the default values, you do not need to specify them.

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II. Reading Files

1. File Open

# The open() function returns a file object, which has a read() method for reading the content of the file:
f = open("demofile.txt")
print(f.read())

# Open a file on a different location:
f = open("D:\\myfiles\welcome.txt")

1.1 Using the with statement

with open("demofile.txt") as f:
  print(f.read())

# Then you do not have to worry about closing your files, the with statement takes care of that.

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2. Close Files

It is a good practice to always close the file when you are done with it.

If you are not using the with statement, you must write a close statement in order to close the file:

f = open("demofile.txt")
print(f.readline())
f.close()

3. Read Only Parts of the File

By default the read() method returns the whole text, but you can also specify how many characters you want to return:

# Return the 5 first characters of the file:
with open("demofile.txt") as f:
  print(f.read(5))

3.1 Read Lines

You can return one line by using the readline() method ( the first line ):

with open("demofile.txt") as f:
  print(f.readline())


# By looping through the lines of the file, you can read the whole file, line by line:
with open("demofile.txt") as f:
  for x in f:
    print(x)

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III. Writing files

1. Write to an Existing File

To write to an existing file, you must add a parameter to the open() function:

• "a" - Append - will append to the end of the file
• "w" - Write - will overwrite any existing content

with open("demofile.txt", "a") as f:
  f.write("Now the file has more content!")

#open and read the file after the appending:
with open("demofile.txt") as f:
  print(f.read())

2. Overwrite Existing Content

To overwrite the existing content to the file, use the "w" parameter:

Note: the "w" method will overwrite the entire file.

with open("demofile.txt", "w") as f:
  f.write("Woops! I have deleted the content!")

#open and read the file after the overwriting:
with open("demofile.txt") as f:
  print(f.read())

3. Create a New File

To create a new file in Python, use the open() method, with one of the following parameters:

• "x" - Create - will create a file, returns an error if the file exists
• "a" - Append - will create a file if the specified file does not exists
• "w" - Write - will create a file if the specified file does not exists

f = open("myfile.txt", "x") # Result: a new empty file is created.

Note: If the file already exist, an error will be raised.

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IV. Pickled files

Pickle files are Python’s way of putting objects to sleep and waking them up later ( Think of pickle as a memory snapshot, not a data exchange format. )

Pickling is serialization: Python converts an object (lists, dicts, classes, models, etc.) into a byte stream and saves it to a file (.pkl).

Unpickling does the reverse: it reconstructs the exact object in memory.

import pickle

# save
with open("data.pkl", "wb") as f:
    pickle.dump(obj, f)

# load
with open("data.pkl", "rb") as f:
    obj = pickle.load(f)

Important truths:

• Pickle is Python-only (not portable across languages).
• It preserves object structure and state, not just data.
• Never unpickle untrusted files → it can execute arbitrary code.

A pickle file is not “just data”. It’s a program for rebuilding an object.