Null Safe Python

Null safe support for Python.

Installation

pip install nullsafe

Quick Start

Dummy Class

class Dummy:
    pass

Normal Python code:

o = Dummy()

try:
    value = o.inexistent
    print("accessed")
except AttributeError:
    value = None

With nullsafe:

from nullsafe import undefined, _

o = Dummy()

value = _(o).inexistent

if value is not undefined:
    print("accessed")

Documentation

Basics

There are 5 values importable in nullsafe root:

class NullSafeProxy: (o: T)

Receives an object o on instantiation.

Proxy class for granting nullsafe abilities to an object.

class NullSafe: ()

No argument needed.

Nullish class with with nullsafe abilities. Instances will have a falsy boolean evaluation, equity comparison (==) to None and instance of NullSafe returns True, otherwise False. Identity comparison (is) to None will return False.

variable undefined: NullSafe

Instance of Nullsafe, this instance will be returned for all nullish access in a proxied object, enabling identity comparison value is undefined for code clarity.

function nullsafe: (o: T) -> T

Receives an object o as argument.

Helper function that checks if object is nullish and return the proxied object.

return undefined if o is None or undefined, otherwise return the proxied object NullSafeProxy[T].

This function is generic typed ((o: T) -> T), code autocompletions and linters functionalities will remain. Disclaimer: If the object was not typed before proxy, it obviously won't come out typed out of the blue.

function _: (o: T) -> T (alias to nullsafe)

Alias to nullsafe, used for better code clarity.

The examples shown will be using _ instead of nullsafe for code clarity. For better understanding, the Javascript equivalents will be shown as comments.

Implementation

Nullsafe abilities are granted after proxying an object through NullSafeProxy. To proxy an object pass it through _() or nullsafe(). Due to language limitation, the implementation does not follow the "return the first nullish value in chain", instead it "extend the a custom nullish value until the end of chain". Inexistent values of a proxied object and its subsequent values in chain will return undefined.

Import

from nullsafe import undefined, _

Usage

There are various way to get a nullsafe proxied object.

Null safe attribute access

Proxied object doing a possibly AttributeError access.

o = Dummy()

# o.inexistent
assert _(o).inexistent is undefined
assert _(o).inexistent == None   # undefined == None
assert not _(o).inexistent       # bool(undefined) == False

# o.inexistent?.nested
assert _(o).inexistent.nested is undefined

# o.existent.inexistent?.nested
assert _(o.existent).inexistent.nested is undefined

# o.maybe?.inexistent?.nested
assert _(_(o).maybe).inexistent.nested is undefined

Null safe item access

Proxied object doing a possibly KeyError access.

o = Dummy() # dict works too !

# o.inexistent
assert _(o)["inexistent"] is undefined
assert _(o)["inexistent"] == None    # undefined == None
assert not _(o)["inexistent"]        # bool(undefined) == False

# o.inexistent?.nested
assert _(o)["inexistent"]["nested"] is undefined

# o.existent.inexistent?.nested
assert _(o["existent"])["inexistent"]["nested"] is undefined

# o.maybe?.inexistent?.nested
assert _(_(o)["maybe"])["inexistent"]["nested"] is undefined

Null safe post evaluation

Possibly None or undefined object doing possibly AttributeError or KeyError access.

Note: This only works if the seeking value is accessible, see limitations

o = Dummy() # dict works too !
o.nay = None

# o.nay?.inexistent
assert _(o.nay).inexistent is undefined
assert _(o.nay).inexistent == None   # undefined == None
assert not _(o.nay).inexistent       # bool(undefined) == False

# o.nay?.inexistent.nested
assert _(o.nay).inexistent.nested is undefined

o = Dummy() # dict works too !
o["nay"] = None

# o.nay?.inexistent
assert _(o["nay"])["inexistent"] is undefined
assert not _(o["nay"])["inexistent"]

# o.nay?.inexistent.nested
assert _(o["nay"])["inexistent"]["nested"] is undefined
assert not _(o["nay"])["inexistent"]["nested"]

Combined usage

Of course you can combine different styles.

assert _(o).inexistent["inexistent"].inexistent.inexistent["inexistent"]["inexistent"] is undefined

Limitations

List of limitations that you may encounter.

undefined behavior

undefined is actually an instance of NullSafe, the actual mechanism used for nullsafe chaining, it cannot self rip the nullsafe functionality when the chain ends (because it doesn't know), so this following actually possible and probably not the wanted behavior.

val = _(o).inexistent

assert val.another_inexistent is undefined

Post evaluation

In other languages like Javascript, it checks for each item in the chain and return undefined on the first nullish value, which in fact is post-evaluated. This is not possible in python because it raises an AttributeError or KeyError on access attempt, unless it returns None (see one of the available usage), so it must proxy the instance that may contain the attr or key before accessing.

try:
    val = _(o.inexistent).nested # AttributeError: '<type>' object has no attribute 'inexistent'
except AttributeError:
    assert True
assert _(o).inexistent.nested is undefined

GitHub

https://github.com/paaksing/nullsafe-python