Python Enum aliases: iteration and member lookup have different domains

Operation contract

The receipt status definition keeps NEW as an alias of PENDING. Ordinary iteration returns each canonical member once, while __members__ contains every declared name, including NEW. Reading the alias name returns the canonical member whose name is PENDING. That matters when a received value must round-trip through a stored status: aliases preserve lookup compatibility but do not preserve the original spelling.

Failure and ownership boundary

Use unique when duplicate values are a schema mistake rather than an intended compatibility policy. Enum aliases are distinct from two external strings mapped to one field by a parser. Do not serialize the Python class name as an enduring wire identifier. Python Enum: parse wire values into a closed state set, Python JSON validation: reject duplicate members and non-integer amounts and Python dictionaries: insertion order and duplicate-key replacement remain separate concerns.

Working program

from enum import Enum, unique

class ReceiptStatus(Enum):
    PENDING = "pending"
    NEW = "pending"
    POSTED = "posted"

print("iteration:", [member.name for member in ReceiptStatus])
print("declared:", list(ReceiptStatus.__members__))
print("alias identity:", ReceiptStatus.NEW is ReceiptStatus.PENDING)
print("alias canonical name:", ReceiptStatus["NEW"].name)
try:
    unique(ReceiptStatus)
except ValueError:
    print("duplicate values rejected")

Output

iteration: ['PENDING', 'POSTED']
declared: ['PENDING', 'NEW', 'POSTED']
alias identity: True
alias canonical name: PENDING
duplicate values rejected

Costs and limits

Lookup uses the declared member mapping. Iteration visits canonical members, so counts differ when aliases exist. Creating a large enum and its mappings has construction and retained-object costs; this two-value definition is not a benchmark.

Common Mistakes

• Iteration omits aliases; use __members__ when examining declared names.
• An alias cannot retain an independently chosen canonical name.

Connected lessons

Python Enum: parse wire values into a closed state set, Python JSON validation: reject duplicate members and non-integer amounts, Python hash and equality: immutable dictionary keys.