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Constants in Python

Python has no true immutable constants at the language level, but conventions and typing help communicate intent.

Naming convention

Use UPPER_CASE identifiers to indicate constants. Tools and peers recognize this convention.

PI = 3.14159
API_URL = "https://api.example.com"
WELCOME = "Hello"

typing.Final

Annotate names with Final to signal they shouldn’t be reassigned (checked by static type checkers like mypy).

from typing import Final

MAX_RETRIES: Final = 3
TIMEOUT_S: Final[float] = 2.5

Reassignments may be flagged by linters/type checkers.

Immutability of objects

Even if a name is constant, the referenced object may be mutable. Prefer immutable types or protect mutations.

NAMES = ("Ada", "Guido")           # tuple (immutable)
CONFIG = {"debug": False}           # mutable mapping
# CONFIG["debug"] = True  # allowed; consider Mapping or dataclasses.replace

Modules as namespaces

Group related constants in modules to keep code organized.

# constants.py
DEFAULT_ENCODING = "utf-8"
BUFFER_SIZE = 8192

Import explicitly to clarify usage:

from constants import DEFAULT_ENCODING

Enums for constrained values

Use enum.Enum when a name must be one of a small set of values with type safety.

from enum import Enum
class Status(Enum):
    PENDING = "pending"
    RUNNING = "running"
    DONE = "done"

Summary

  • Use UPPER_CASE and typing.Final to mark constants
  • Prefer immutable objects or protect mutable ones
  • Consider Enum for closed sets of allowed values; modules act as namespaces
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