How to Declare and Use Constants in Python

Unwritten Rules

In Python, there isn't a specific syntax to declare constants; instead, the convention is to use all uppercase variable names to indicate that they should be treated as constants. For example, PI = 3.14 suggests that PI is a constant value. However, this is just a convention and doesn't prevent the variable from being altered at runtime.

Why use uppercase? This practice helps other programmers understand the intent that the variable should remain unchanged throughout the code. It's a common unwritten rule that improves code readability and maintainability.

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Frozen Dataclasses

To encapsulate constant values in Python, you can leverage frozen dataclasses. A frozen dataclass is a Python dataclass where the instances are immutable. This can be quite useful when you want to ensure that once a value is set, it cannot be changed. Here’s a quick example:


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from dataclasses import dataclass

@dataclass(frozen=True)
class Constants:
    PI: float = 3.14159
    MAX_CONNECTIONS: int = 100

By using a frozen dataclass, you define a structure that inherently discourages modification of its fields, aligning with the principle of constants.

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Use of Libraries

For those seeking a more robust solution, the pconst library offers a way to declare constants with a special type. The library enforces immutability and prevents reassignment of values once set, which is absent from native Python. To use it, first install it via pip install pconst and then define constants like so:


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from pconst import const

const.PI = 3.14159
const.MAX_CONNECTIONS = 100

Using such a library can enhance code integrity by programmatically restricting changes to these variables after their initial assignment.

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Practical Examples

Here's how you might use constants effectively in Python:

Example 1: Default Values

When writing functions, you can use constants for default argument values:


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DEFAULT_DB_PATH = "/path/to/database.sqlite"

def connect_to_db(path=DEFAULT_DB_PATH):
    # Connection logic here
    pass

Example 2: Configuration

Constants can be used to manage configuration settings:


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CONFIG = {
    'MAX_USERS': 10,
    'TIMEOUT': 300
}

def configure_app(config=CONFIG):
    # Apply configuration
    pass

These examples demonstrate constants in action, providing clarity and flexibility when managing default settings across your codebase.

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How to Declare and Use Constants in Python

Unwritten Rules

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Frozen Dataclasses


Copy Code
from dataclasses import dataclass

@dataclass(frozen=True)
class Constants:
    PI: float = 3.14159
    MAX_CONNECTIONS: int = 100

By using a frozen dataclass, you define a structure that inherently discourages modification of its fields, aligning with the principle of constants.

Expand

Use of Libraries


Copy Code
from pconst import const

const.PI = 3.14159
const.MAX_CONNECTIONS = 100

Using such a library can enhance code integrity by programmatically restricting changes to these variables after their initial assignment.

Expand

Practical Examples

Here's how you might use constants effectively in Python:

Example 1: Default Values

When writing functions, you can use constants for default argument values:


Copy Code
DEFAULT_DB_PATH = "/path/to/database.sqlite"

def connect_to_db(path=DEFAULT_DB_PATH):
    # Connection logic here
    pass

Example 2: Configuration

Constants can be used to manage configuration settings:


Copy Code
CONFIG = {
    'MAX_USERS': 10,
    'TIMEOUT': 300
}

def configure_app(config=CONFIG):
    # Apply configuration
    pass

These examples demonstrate constants in action, providing clarity and flexibility when managing default settings across your codebase.

Expand