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Python Enhancement Proposals

PEP 671 – Syntax for late-bound function argument defaults

Chris Angelico <rosuav at>
Python-Ideas thread
Standards Track
24-Oct-2021, 01-Dec-2021

Table of Contents


Function parameters can have default values which are calculated during function definition and saved. This proposal introduces a new form of argument default, defined by an expression to be evaluated at function call time.


Optional function arguments, if omitted, often have some sort of logical default value. When this value depends on other arguments, or needs to be reevaluated each function call, there is currently no clean way to state this in the function header.

Currently-legal idioms for this include:

# Very common: Use None and replace it in the function
def bisect_right(a, x, lo=0, hi=None, *, key=None):
    if hi is None:
        hi = len(a)

# Also well known: Use a unique custom sentinel object
def connect(timeout=_USE_GLOBAL_DEFAULT):
    if timeout is _USE_GLOBAL_DEFAULT:
        timeout = default_timeout

# Unusual: Accept star-args and then validate
def add_item(item, *optional_target):
    if not optional_target:
        target = []
        target = optional_target[0]

In each form, help(function) fails to show the true default value. Each one has additional problems, too; using None is only valid if None is not itself a plausible function parameter, the custom sentinel requires a global constant; and use of star-args implies that more than one argument could be given.


Function default arguments can be defined using the new => notation:

def bisect_right(a, x, lo=0, hi=>len(a), *, key=None):
def connect(timeout=>default_timeout):
def add_item(item, target=>[]):
def format_time(fmt, time_t=>time.time()):

The expression is saved in its source code form for the purpose of inspection, and bytecode to evaluate it is prepended to the function’s body.

Notably, the expression is evaluated in the function’s run-time scope, NOT the scope in which the function was defined (as are early-bound defaults). This allows the expression to refer to other arguments.

Multiple late-bound arguments are evaluated from left to right, and can refer to previously-defined values. Order is defined by the function, regardless of the order in which keyword arguments may be passed.

def prevref(word=”foo”, a=>len(word), b=>a//2): # Valid def selfref(spam=>spam): # UnboundLocalError def spaminate(sausage=>eggs + 1, eggs=>sausage - 1): # Confusing, don’t do this def frob(n=>len(items), items=[]): # See below

Evaluation order is left-to-right; however, implementations MAY choose to do so in two separate passes, first for all passed arguments and early-bound defaults, and then a second pass for late-bound defaults. Otherwise, all arguments will be assigned strictly left-to-right.

Rejected choices of spelling

While this document specifies a single syntax name=>expression, alternate spellings are similarly plausible. The following spellings were considered:

def bisect(a, hi=>len(a)):
def bisect(a, hi:=len(a)):
def bisect(a, hi?=len(a)):
def bisect(a, @hi=len(a)):

Since default arguments behave largely the same whether they’re early or late bound, the chosen syntax hi=>len(a) is deliberately similar to the existing early-bind syntax.

One reason for rejection of the := syntax is its behaviour with annotations. Annotations go before the default, so in all syntax options, it must be unambiguous (both to the human and the parser) whether this is an annotation, a default, or both. The alternate syntax target:=expr runs the risk of being misinterpreted as target:int=expr with the annotation omitted in error, and may thus mask bugs. The chosen syntax target=>expr does not have this problem.

How to Teach This

Early-bound default arguments should always be taught first, as they are the simpler and more efficient way to evaluate arguments. Building on them, late bound arguments are broadly equivalent to code at the top of the function:

def add_item(item, target=>[]):

# Equivalent pseudocode:
def add_item(item, target=<OPTIONAL>):
    if target was omitted: target = []

A simple rule of thumb is: “target=expression” is evaluated when the function is defined, and “target=>expression” is evaluated when the function is called. Either way, if the argument is provided at call time, the default is ignored. While this does not completely explain all the subtleties, it is sufficient to cover the important distinction here (and the fact that they are similar).

Interaction with other proposals

PEP 661 attempts to solve one of the same problems as this does. It seeks to improve the documentation of sentinel values in default arguments, where this proposal seeks to remove the need for sentinels in many common cases. PEP 661 is able to improve documentation in arbitrarily complicated functions (it cites traceback.print_exception as its primary motivation, which has two arguments which must both-or-neither be specified); on the other hand, many of the common cases would no longer need sentinels if the true default could be defined by the function. Additionally, dedicated sentinel objects can be used as dictionary lookup keys, where PEP 671 does not apply.

A generic system for deferred evaluation has been proposed at times (not to be confused with PEP 563 and PEP 649 which are specific to annotations). While it may seem, on the surface, that late-bound argument defaults are of a similar nature, they are in fact unrelated and orthogonal ideas, and both could be of value to the language. The acceptance or rejection of this proposal would not affect the viability of a deferred evaluation proposal, and vice versa. (A key difference between generalized deferred evaluation and argument defaults is that argument defaults will always and only be evaluated as the function begins executing, whereas deferred expressions would only be realized upon reference.)

Implementation details

The following relates to the reference implementation, and is not necessarily part of the specification.

Argument defaults (positional or keyword) have both their values, as already retained, and an extra piece of information. For positional arguments, the extras are stored in a tuple in __defaults_extra__, and for keyword-only, a dict in __kwdefaults_extra__. If this attribute is None, it is equivalent to having None for every argument default.

For each parameter with a late-bound default, the special value Ellipsis is stored as the value placeholder, and the corresponding extra information needs to be queried. If it is None, then the default is indeed the value Ellipsis; otherwise, it is a descriptive string and the true value is calculated as the function begins.

When a parameter with a late-bound default is omitted, the function will begin with the parameter unbound. The function begins by testing for each parameter with a late-bound default using a new opcode QUERY_FAST/QUERY_DEREF, and if unbound, evaluates the original expression. This opcode (available only for fast locals and closure variables) pushes True onto the stack if the given local has a value, and False if not - meaning that it pushes False if LOAD_FAST or LOAD_DEREF would raise UnboundLocalError, and True if it would succeed.

Out-of-order variable references are permitted as long as the referent has a value from an argument or early-bound default.


When no late-bound argument defaults are used, the following costs should be all that are incurred:

  • Function objects require two additional pointers, which will be NULL
  • Compiling code and constructing functions have additional flag checks
  • Using Ellipsis as a default value will require run-time verification to see if late-bound defaults exist.

These costs are expected to be minimal (on 64-bit Linux, this increases all function objects from 152 bytes to 168), with virtually no run-time cost when late-bound defaults are not used.

Backward incompatibility

Where late-bound defaults are not used, behaviour should be identical. Care should be taken if Ellipsis is found, as it may not represent itself, but beyond that, tools should see existing code unchanged.



Last modified: 2022-06-21 01:02:35 GMT