Python Context Variables

Context variables can have different values depending on their context. Unlike Thread-Local Storage where each execution thread may have a different value for a variable, a context variable may have several contexts in one execution thread. This is useful for keeping track of variables in concurrent asynchronous tasks.

The ContextVar class is used to declare and work with Context Variables.

Creating a Context Variable

You can create a context variable with an optional default value ?

import contextvars

name = contextvars.ContextVar("name", default="Hello")
print(f"Variable name: {name.name}")
print(f"Default value: {name.get()}")

Variable name: name
Default value: Hello

The optional default parameter is returned by ContextVar.get() when no value for the variable is found in the current context.

ContextVar Methods

Using get() and set()

The get() method retrieves the current value, while set() assigns a new value ?

import contextvars

name = contextvars.ContextVar("name", default="World")

# Get current value
print(f"Initial: {name.get()}")

# Set a new value
token = name.set("Python")
print(f"After set: {name.get()}")

Initial: World
After set: Python

Using reset()

The reset() method restores the previous value using a token ?

import contextvars

name = contextvars.ContextVar("name", default="World")

print(f"Original: {name.get()}")
token = name.set("Python")
print(f"After set: {name.get()}")

# Reset to previous value
name.reset(token)
print(f"After reset: {name.get()}")

Original: World
After set: Python
After reset: World

Working with Context Objects

The Context class is a mapping of Context Variables to their values. You can create contexts and run code within them ?

import contextvars

name = contextvars.ContextVar("name")
age = contextvars.ContextVar("age")

# Set values in current context
name.set("Alice")
age.set(25)

# Create a new context
ctx = contextvars.Context()
print(f"Empty context length: {len(ctx)}")

# Copy current context
current_ctx = contextvars.copy_context()
print(f"Current context length: {len(current_ctx)}")
print(f"Variables: {list(current_ctx.keys())}")

Empty context length: 0
Current context length: 2
Variables: [<ContextVar name='name' at 0x...>, <ContextVar name='age' at 0x...>]

Running Code in Different Contexts

You can execute functions within specific contexts using the run() method ?

import contextvars

name = contextvars.ContextVar("name", default="Unknown")

def print_name():
    return f"Name: {name.get()}"

# Set value in current context
name.set("Alice")
print("Current context:", print_name())

# Create new context and run function
new_ctx = contextvars.Context()
result = new_ctx.run(print_name)
print("New context:", result)

# Original context unchanged
print("Back to current:", print_name())

Current context: Name: Alice
New context: Name: Unknown
Back to current: Name: Alice

Context Variable Methods Summary

Context Object Methods Summary

Conclusion

Context variables provide isolated storage for values across different execution contexts. Use ContextVar for declaring variables and Context objects for managing execution environments in asynchronous applications.