A module implementing custom literal suffixes using pure Python. custom-literals mimics C++’s user-defined literals (UDLs) by defining literal suffixes that can be accessed as attributes of literal values, such as numeric constants, string literals and more.

(c) RocketRace 2022-present. See LICENSE file for more.


See the examples/ directory for more.

Function decorator syntax:

from custom_literals import literal
from datetime import timedelta

@literal(float, int, name="s")
def seconds(self):
    return timedelta(seconds=self)

@literal(float, int, name="m")
def minutes(self):
    return timedelta(seconds=60 * self)

print(30 .s + 0.5.m) # 0:01:00

Class decorator syntax:

from custom_literals import literals
from datetime import timedelta

@literals(float, int)
class Duration:
    def s(self):
        return timedelta(seconds=self)
    def m(self):
        return timedelta(seconds=60 * self)

print(30 .s + 0.5.m) # 0:01:00

Removing a custom literal:

from custom_literals import literal, unliteral

def u(self):
    return self.upper()

print("hello".u) # "HELLO"

unliteral(str, "u")
assert not hasattr("hello", "u")

Context manager syntax (automatically removes literals afterwards):

from custom_literals import literally
from datetime import timedelta

with literally(float, int, 
    s=lambda x: timedelta(seconds=x), 
    m=lambda x: timedelta(seconds=60 * x)
    print(30 .s + 0.5.m) # 0:01:00


Currently, three methods of defining custom literals are supported: The function decorator syntax @literal, the class decorator syntax @literals, and the context manager syntax with literally. (The latter will automatically unhook the literal suffixes when the context is exited.) To remove a custom literal, use unliteral.

Custom literals are defined for literal values of the following types:

Type Example Notes
int (42).x The Python parser interprets 42.x as a float literal followed by an identifier. To avoid this, use (42).x or 42 .x instead.
float 3.14.x
complex 1j.x
bool True.x Since bool is a subclass of int, int hooks may influence bool as well.
str "hello".x F-strings (f"{a}".x) are also supported.
bytes b"hello".x
None None.x
Ellipsis ....x Yes, this is valid syntax.
tuple (1, 2, 3).x Generator expressions ((x for x in ...)) are not tuple literals and thus won’t be affected by literal suffixes.
list [1, 2, 3].x List comprehensions ([x for x in ...]) may not function properly.
set {1, 2, 3}.x Set comprehensions ({x for x in ...}) may not function properly.
dict {"a": 1, "b": 2}.x Dict comprehensions ({x: y for x, y in ...}) may not function properly.

In addition, custom literals can be defined to be strict, that is, only allow the given literal suffix to be invoked on constant, literal values. This means that the following code will raise a TypeError:

@literal(str, name="u", strict=True)
def utf_8(self):
    return self.encode("utf-8")

my_string = "hello"
# TypeError: the strict custom literal `u` of `str` objects can only be invoked on literal values

By default, custom literals are not strict. This is because determining whether a suffix was invoked on a literal value relies on bytecode analysis, which is a feature of the CPython interpreter, and is not guaranteed to be forwards compatible. It can be enabled by passing strict=True to the @literal, @literals or literally functions.



This library relies almost entirely on implementation-specific behavior of the CPython interpreter. It is not guaranteed to work on all platforms, or on all versions of Python. It has been tested on common platforms (windows, ubuntu, macos) using python 3.7 through to 3.10, but while changes that would break the library are quite unlikely, they are not impossible either.

That being said, custom_literals does its absolute best to guarantee maximum stability of the library, even in light of possible breaking changes in CPython internals. The code base is well tested. In the future, the library may also expose multiple different backends for the actual implementation of builtin type patching. As of now, the only valid backend is forbiddenfruit, which uses the forbiddenfruit library.

Type safety

The library code, including the public API, is fully typed. Registering and unregistering hooks is type-safe, and static analysis tools should have nothing to complain about.

However, accessing custom literal suffixes is impossible to type-check. This is because all major static analysis tools available for python right now (understandably) assume that builtins types are immutable. That is, the attributes and methods builtin types cannot be dynamically modified. This goes against the core idea of the library, which is to patch custom attributes on builtin types.

Therefore, if you are using linters, type checkers or other static analysis tools, you will likely encounter many warnings and errors. If your tool allows it, you should disable these warnings (ideally on a per-diagnostic, scoped basis) if you want to use this library without false positives.


Should I use this in production?

Emphatically, no! But I won’t stop you.


Python’s operator overloading allows for flexible design of domain-specific languages. However, Python pales in comparison to C++ in this aspect. In particular, User-Defined Literals (UDLs) are a powerful feature of C++ missing in Python. This library is designed to emulate UDLs in Python, with syntactic sugar comparable to C++ in elegance.

But really, why?

Because it’s posssible.

Nooooooo (runs away from computer)

I kind of disagree: yessss (dances in front of computer)

Could this ever be type safe?

I doubt it. The assumptions made by static analysis tools are incredibly useful, and this is such an edge case it makes no sense for them to assume builtin literal types can have dynamically set attributes. In addition, there isn’t a good way to signal to your type checker that an immutable type is going to be endowed with new attributes!


(c) RocketRace 2022-present. This library is under the Mozilla Public License 2.0. See the LICENSE file for more details.


Patches, bug reports, feature requests and pull requests are welcome.



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