Python partial Placeholder: leave a positional amount open

Last updated: 1 Oct 20265 min read

tutorial

functools.Placeholder reserves a positional slot inside a partially applied callable.

Operation contract

A quote function expects an amount, a fixed fee, and a currency. The partial binds the latter two while leaving the amount open, so a caller can price a forty-seven-unit invoice with a seven-unit fee. A call that supplies no amount fails before the quote function runs.

Failure boundary

Every placeholder must receive a positional value at call time. Placeholder cannot stand in for a keyword argument. A partial object binds values but does not validate whether the fee, currency, or amount matches a live pricing policy; that belongs in the called function or its owner.

Working program

from functools import Placeholder, partial

def invoice_balance(amount, fixed_fee, currency):
    return f"{currency} {amount - fixed_fee}"

quote_after_fee = partial(invoice_balance, Placeholder, 7, "INR")
print("balance", quote_after_fee(47))
try:
    quote_after_fee()
except TypeError:
    print("missing_amount_rejected", True)

Output

balance INR 40
missing_amount_rejected True

Costs and limits

A partial stores bound argument references and adds a small call wrapper. It does not precompute the invoice result or clone mutable bound objects.

Common Mistakes

All reserved positional slots must be filled.
Placeholder is not supported as a keyword value in partial.
Binding an amount or fee does not enforce a business rule.

Connected lessons

Test this contract.

python

partial-placeholder-position