bazel-skylib/docs/partial_doc.md

Starlark module for working with partial function objects.

Partial function objects allow some parameters are bound before the call.

Similar to https://docs.python.org/3/library/functools.html#functools.partial.

partial.make

partial.make(func, args, kwargs)

Creates a partial that can be called using call.

A partial can have args assigned to it at the make site, and can have args passed to it at the call sites.

A partial 'function' can be defined with positional args and kwargs:

function with no args

def function1():
  ...

function with 2 args

def function2(arg1, arg2):
  ...

function with 2 args and keyword args

def function3(arg1, arg2, x, y):
  ...

The positional args passed to the function are the args passed into make followed by any additional positional args given to call. The below example illustrates a function with two positional arguments where one is supplied by make and the other by call:

function demonstrating 1 arg at make site, and 1 arg at call site

def _foo(make_arg1, func_arg1):
  print(make_arg1 + " " + func_arg1 + "!")

For example:

hi_func = partial.make(_foo, "Hello")
bye_func = partial.make(_foo, "Goodbye")
partial.call(hi_func, "Jennifer")
partial.call(hi_func, "Dave")
partial.call(bye_func, "Jennifer")
partial.call(bye_func, "Dave")

prints:

"Hello, Jennifer!"
"Hello, Dave!"
"Goodbye, Jennifer!"
"Goodbye, Dave!"

The keyword args given to the function are the kwargs passed into make unioned with the keyword args given to call. In case of a conflict, the keyword args given to call take precedence. This allows you to set a default value for keyword arguments and override it at the call site.

Example with a make site arg, a call site arg, a make site kwarg and a call site kwarg:

def _foo(make_arg1, call_arg1, make_location, call_location):
  print(make_arg1 + " is from " + make_location + " and " +
        call_arg1 + " is from " + call_location + "!")

func = partial.make(_foo, "Ben", make_location="Hollywood")
partial.call(func, "Jennifer", call_location="Denver")

Prints "Ben is from Hollywood and Jennifer is from Denver!".

partial.call(func, "Jennifer", make_location="LA", call_location="Denver")

Prints "Ben is from LA and Jennifer is from Denver!".

Note that keyword args may not overlap with positional args, regardless of whether they are given during the make or call step. For instance, you can't do:

def foo(x):
  pass

func = partial.make(foo, 1)
partial.call(func, x=2)

PARAMETERS

Name	Description	Default Value
func	The function to be called.	none
args	Positional arguments to be passed to function.	none
kwargs	Keyword arguments to be passed to function. Note that these can be overridden at the call sites.	none

RETURNS

A new partial that can be called using call

partial.call

partial.call(partial, args, kwargs)

Calls a partial created using make.

PARAMETERS

Name	Description	Default Value
partial	The partial to be called.	none
args	Additional positional arguments to be appended to the ones given to make.	none
kwargs	Additional keyword arguments to augment and override the ones given to make.	none

RETURNS

Whatever the function in the partial returns.

partial.is_instance

partial.is_instance(v)

Returns True if v is a partial created using make.

PARAMETERS

Name	Description	Default Value
v	The value to check.	none

RETURNS

True if v was created by make, False otherwise.