How to Use Pickle in Python: Save and Load Objects Easily

Use the pickle module in Python to save objects to a file with pickle.dump() and load them back with pickle.load(). This lets you store complex data like lists or dictionaries and retrieve them later exactly as they were.

Syntax

The pickle module provides two main functions for saving and loading objects:

• pickle.dump(obj, file): Saves the object obj to the open file file.
• pickle.load(file): Loads and returns the object from the open file file.

You must open the file in binary mode: 'wb' for writing and 'rb' for reading.

python

import pickle

# Save an object to a file
obj = {'example': 123}  # Added example object to avoid NameError
with open('data.pkl', 'wb') as f:
    pickle.dump(obj, f)

# Load the object from the file
with open('data.pkl', 'rb') as f:
    obj = pickle.load(f)

Example

This example shows how to save a Python dictionary to a file and then load it back exactly as it was.

python

import pickle

# Data to save
my_data = {'name': 'Alice', 'age': 30, 'scores': [85, 90, 92]}

# Save data to file
with open('my_data.pkl', 'wb') as file:
    pickle.dump(my_data, file)

# Load data from file
with open('my_data.pkl', 'rb') as file:
    loaded_data = pickle.load(file)

print(loaded_data)

Output

{'name': 'Alice', 'age': 30, 'scores': [85, 90, 92]}

Common Pitfalls

Common mistakes when using pickle include:

• Not opening the file in binary mode ('wb' or 'rb'), which causes errors.
• Trying to pickle objects that are not supported, like open file handles or database connections.
• Loading pickled data from untrusted sources, which can be a security risk.

Always use pickle only with trusted data and open files correctly.

python

import pickle

# Wrong: opening file in text mode causes error
try:
    with open('data.pkl', 'w') as f:
        pickle.dump({'a': 1}, f)
except Exception as e:
    print('Error:', e)

# Right: open file in binary mode
with open('data.pkl', 'wb') as f:
    pickle.dump({'a': 1}, f)

Output

Error: write() argument must be str, not bytes

Quick Reference

Remember these key points when using pickle:

• Use pickle.dump(obj, file) to save objects.
• Use pickle.load(file) to load objects.
• Always open files in binary mode: 'wb' for writing, 'rb' for reading.
• Only unpickle data you trust to avoid security risks.

Key Takeaways

Use pickle.dump() to save Python objects to a binary file.

Use pickle.load() to read objects back from the file.

Always open files in binary mode ('wb' for writing, 'rb' for reading).

Do not unpickle data from untrusted sources to avoid security risks.

Pickle can save complex Python objects like lists and dictionaries easily.