novalue

final class pythonic_fp.gadgets.sentinels.novalue.NoValue

Bases: object

missing value

Singleton class representing an actual, not potential, missing value.

While None and () are frequently used as sentinel values, I prefer to think of them as

• None as returns, or returned, no values.

• () as an empty, possibly typed, iterable collection.

Important

Given variables

x: int | NoValue
y: int | NoValue

Equality between x and y means both values exist and compare as equal.

x == y

	y = NoValue()	y = 42	y = 57
x = NoValue()	False	False	False
x = 42	False	True	False
x = 57	False	False	True

x != y

	y = NoValue()	y = 42	y = 57
x = NoValue()	False	False	False
x = 42	False	False	True
x = 57	False	True	False

Warning

• use == or != only in value comparisons

• use is and is not to identity the NoValue() singleton itself

Tip

Use in a union type when creating “optional” arguments to functions and methods.

To help ensure the abstraction stays a hidden implementation detail and does not leak out into user code,

• Do not export the sentinel value.

  • A new reference can always be generated via NoValue().

• Use @overload to keep the NoValue type out of documentation and IDEs.

static __new__(cls) → NoValue

new

returns:

The NoValue() singleton instance.

__reduce__() → tuple[type[NoValue], tuple[()]]

reduce

Tell pickle and copy how to reconstruct the sentinel: call Sentinel(flavor) rather than rebuilding the object directly.

• reconstruction goes through __new__, so the flavor cache is consulted

• unpickling returns the existing NoValue singleton, or creates and caches it in a fresh interpreter

• copy and deepcopy use the same protocol, so they return the singleton unchanged

returns:

The pair (NoValue, ()), which pickle calls to rebuild the sentinel.

Important

Without this, pickle would construct a duplicate NoValue outside the flavor cache, and is comparisons against it would silently fail.

__hash__() → int

hash

returns:

The singleton’s unique integer hash value.

__eq__(other: object) → bool

Equality comparison

param other:

The object to be compared.

returns:

False even if compared to itself.

Warning

• non-standard comparison semantics

• always returns False

• if one or both values are missing, then what is there to compare?

__repr__() → str

repr string

returns:

The string ‘NoValue()’.

__bool__() → bool

bool

Always falsy.

returns:

False

__ne__(other: object) → bool

not equal

returns:

False

Warning

• non-standard comparison semantics

• always returns False