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Starlark APIs

False​

False: bool

None​

None: None

True​

True: bool

abs​

def abs(
x: float | int,
/,
) -> float | int

Take the absolute value of an int.

abs(0)   == 0
abs(-10) == 10
abs(10)  == 10
abs(10.0) == 10.0
abs(-12.34) == 12.34

all​

def all(x: typing.Iterable, /) -> bool

all: returns true if all values in the iterable object have a truth value of true.

all([1, True]) == True
all([1, 1]) == True
all([0, 1, True]) == False
all([True, 1, True]) == True
all([0, 0]) == False
all([0, False]) == False
all([True, 0]) == False
all([1, False]) == False

any​

def any(x: typing.Iterable, /) -> bool

any: returns true if any value in the iterable object have a truth value of true.

any([0, True]) == True
any([0, 1]) == True
any([0, 1, True]) == True
any([0, 0]) == False
any([0, False]) == False

breakpoint​

def breakpoint() -> None

When a debugger is available, breaks into the debugger.

call_stack​

def call_stack(
*,
strip_frames: int = 0,
) -> str

Get a textual representation of the call stack.

This is intended only for debugging purposes to display to a human and should not be considered stable or parseable.

strip_frames will pop N frames from the top of the call stack, which can be useful to hide non-interesting lines - for example, strip_frames=1 will hide the call to and location of call_stack() itself.

call_stack_frame​

def call_stack_frame(
n: int,
/,
) -> None | StackFrame

Get a structural representation of the n-th call stack frame.

With n=0 returns call_stack_frame itself. Returns None if n is greater than or equal to the stack size.

chr​

def chr(
i: int,
/,
) -> str

chr: returns a string encoding a codepoint.

chr(i) returns a string that encodes the single Unicode code point whose value is specified by the integer i. chr fails unless 0 ≤ i ≤ 0x10FFFF.

chr(65) == 'A'
chr(1049) == 'Й'
chr(0x1F63F) == '😿'

debug​

def debug(val, /) -> str

Print the value with full debug formatting. The result may not be stable over time. Intended for debugging purposes and guaranteed to produce verbose output not suitable for user display.

dir​

def dir(x, /) -> list[str]

dir: list attributes of a value.

dir(x) returns a list of the names of the attributes (fields and methods) of its operand. The attributes of a value x are the names f such that x.f is a valid expression.

"capitalize" in dir("abc")

enum​

def enum(*args: str)

The enum type represents one value picked from a set of values.

For example:

MyEnum = enum("option1", "option2", "option3")

This statement defines an enumeration MyEnum that consists of the three values "option1", "option2" and option3.

Now MyEnum is defined, it's possible to do the following:

  • Create values of this type with MyEnum("option2"). It is a runtime error if the argument is not one of the predeclared values of the enumeration.
  • Get the type of the enum suitable for a type annotation with MyEnum.
  • Given a value of the enum (for example, v = MyEnum("option2")), get the underlying value v.value == "option2" or the index in the enumeration v.index == 1.
  • Get a list of the values that make up the array with MyEnum.values() == ["option1", "option2", "option3"].
  • Treat MyEnum a bit like an array, with len(MyEnum) == 3, MyEnum[1] == MyEnum("option2") and iteration over enums [x.value for x in MyEnum] == ["option1", "option2", "option3"].

Enumeration types store each value once, which are then efficiently referenced by enumeration values.

enumerate​

def enumerate(
it: typing.Iterable,
/,
start: int = 0,
) -> list[(int, typing.Any)]

enumerate: return a list of (index, element) from an iterable.

enumerate(x) returns a list of (index, value) pairs, each containing successive values of the iterable sequence and the index of the value within the sequence.

The optional second parameter, start, specifies an integer value to add to each index.

enumerate(["zero", "one", "two"]) == [(0, "zero"), (1, "one"), (2, "two")]
enumerate(["one", "two"], 1) == [(1, "one"), (2, "two")]

eval_type​

def eval_type(ty: type, /) -> type

Create a runtime type object which can be used to check if a value matches the given type.

fail​

def fail(*args) -> typing.Never

fail: fail the execution

fail("this is an error")  # fail: this is an error
fail("oops", 1, False)  # fail: oops 1 False

field​

def field(typ, /, default = ...) -> field

Creates a field record. Used as an argument to the record function.

rec_type = record(host=field(str), port=field(int), mask=field(int, default=255))
rec = rec_type(host="localhost", port=80)
rec.port == 80
rec.mask == 255

filter​

def filter(func: None | typing.Callable, seq: typing.Iterable, /) -> list

Apply a predicate to each element of the iterable, returning those that match. As a special case if the function is None then removes all the None values.

filter(bool, [0, 1, False, True]) == [1, True]
filter(lambda x: x > 2, [1, 2, 3, 4]) == [3, 4]
filter(None, [True, None, False]) == [True, False]

getattr​

def getattr(
a,
attr: str,
default = ...,
/,
)

getattr: returns the value of an attribute

getattr(x, name) returns the value of the attribute (field or method) of x named name. It is a dynamic error if x has no such attribute.

getattr(x, "f") is equivalent to x.f.

getattr(x, "f", d) is equivalent to x.f if hasattr(x, "f") else d and will never raise an error.

getattr("banana", "split")("a") == ["b", "n", "n", ""] # equivalent to "banana".split("a")

hasattr​

def hasattr(
a,
attr: str,
/,
) -> bool

hasattr: test if an object has an attribute

hasattr(x, name) reports whether x has an attribute (field or method) named name.

hash​

def hash(
a: str,
/,
) -> int

hash: returns the hash number of a value.

hash(x) returns an integer hash value for x such that x == y implies hash(x) == hash(y).

hash fails if x, or any value upon which its hash depends, is unhashable.

hash("hello") != hash("world")

isinstance​

def isinstance(
value,
ty: type,
/,
) -> bool

Check if a value matches the given type.

This operation can be very fast or very slow depending on how it is used.

isinstance(x, list) is very fast, because it is compiled to a special bytecode instruction.

isinstance(x, list[str]) is O(N) operation because it checks every element in this list.

L = list; [isinstance(x, L) for x in y] is slow when L is not a constant: isinstance() first converts list to a type in a loop, which is slow.

But last operation can be optimized like this: L = eval_type(list); [isinstance(x, L) for x in y]: eval_type() converts list value into prepared type matcher.

len​

def len(a, /) -> int

len: get the length of a sequence

len(x) returns the number of elements in its argument.

It is a dynamic error if its argument is not a sequence.

len(()) == 0
len({}) == 0
len([]) == 0
len([1]) == 1
len([1,2]) == 2
len({'16': 10}) == 1
len(True)    # error: not supported

map​

def map(func: typing.Callable, seq: typing.Iterable, /) -> list

Apply a function to each element of the iterable, returning the results.

map(abs, [7, -5, -6]) == [7, 5, 6]
map(lambda x: x * 2, [1, 2, 3, 4]) == [2, 4, 6, 8]

max​

def max(*args, key = ...)

max: returns the maximum of a sequence.

max(x) returns the greatest element in the iterable sequence x.

It is an error if any element does not support ordered comparison, or if the sequence is empty.

The optional named parameter key specifies a function to be applied to each element prior to comparison.

max([3, 1, 4, 1, 5, 9])               == 9
max("two", "three", "four")           == "two"    # the lexicographically greatest
max("two", "three", "four", key=len)  == "three"  # the longest

min​

def min(*args, key = ...)

min: returns the minimum of a sequence.

min(x) returns the least element in the iterable sequence x.

It is an error if any element does not support ordered comparison, or if the sequence is empty.

min([3, 1, 4, 1, 5, 9])                 == 1
min("two", "three", "four")             == "four"  # the lexicographically least
min("two", "three", "four", key=len)    == "two"   # the shortest

ord​

def ord(
a: str,
/,
) -> int

ord: returns the codepoint of a character

ord(s) returns the integer value of the sole Unicode code point encoded by the string s.

If s does not encode exactly one Unicode code point, ord fails. Each invalid code within the string is treated as if it encodes the Unicode replacement character, U+FFFD.

Example:

ord("A")                                == 65
ord("Й")                                == 1049
ord("😿")                               == 0x1F63F

partial​

def partial(func, /, *args, **kwargs) -> function

Construct a partial application. In almost all cases it is simpler to use a lamdba.

pprint​

def pprint(*args) -> None

prepr​

def prepr(a, /) -> str

Like repr, but produces more verbose pretty-printed output

print​

def print(*args) -> None

Print some values to the output.

pstr​

def pstr(a, /) -> str

Like str, but produces more verbose pretty-printed output

record​

def record(**kwargs) -> function

A record type represents a set of named values, each with their own type.

For example:

MyRecord = record(host=str, port=int)

This above statement defines a record MyRecord with 2 fields, the first named host that must be of type str, and the second named port that must be of type int.

Now MyRecord is defined, it's possible to do the following:

  • Create values of this type with MyRecord(host="localhost", port=80). It is a runtime error if any arguments are missed, of the wrong type, or if any unexpected arguments are given.
  • Get the type of the record suitable for a type annotation with MyRecord.type.
  • Get the fields of the record. For example, v = MyRecord(host="localhost", port=80) will provide v.host == "localhost" and v.port == 80. Similarly, dir(v) == ["host", "port"].

It is also possible to specify default values for parameters using the field function.

For example:

MyRecord = record(host=str, port=field(int, 80))

Now the port field can be omitted, defaulting to 80 is not present (for example, MyRecord(host="localhost").port == 80).

Records are stored deduplicating their field names, making them more memory efficient than dictionaries.

repr​

def repr(a, /) -> str

repr: formats its argument as a string.

All strings in the result are double-quoted.

repr(1)                 == '1'
repr("x")               == "\"x\""
repr([1, "x"])          == "[1, \"x\"]"
repr("test \"'")        == "\"test \\\"'\""
repr("x\"y😿 \\'")      == "\"x\\\"y\\U0001f63f \\\\'\""

reversed​

def reversed(a: typing.Iterable, /) -> list

reversed: reverse a sequence

reversed(x) returns a new list containing the elements of the iterable sequence x in reverse order.

reversed(['a', 'b', 'c'])              == ['c', 'b', 'a']
reversed(range(5))                     == [4, 3, 2, 1, 0]
reversed("stressed".elems())           == ["d", "e", "s", "s", "e", "r", "t", "s"]
reversed({"one": 1, "two": 2}.keys())  == ["two", "one"]

sorted​

def sorted(
x: typing.Iterable,
/,
*,
key = ...,
reverse: bool = False,
) -> list

sorted: sort a sequence

sorted(x) returns a new list containing the elements of the iterable sequence x, in sorted order. The sort algorithm is stable.

The optional named parameter reverse, if true, causes sorted to return results in reverse sorted order.

The optional named parameter key specifies a function of one argument to apply to obtain the value's sort key. The default behavior is the identity function.

sorted([3, 1, 4, 1, 5, 9])                               == [1, 1, 3, 4, 5, 9]
sorted([3, 1, 4, 1, 5, 9], reverse=True)                 == [9, 5, 4, 3, 1, 1]
sorted(["two", "three", "four"], key=len)                == ["two", "four", "three"] # shortest to longest
sorted(["two", "three", "four"], key=len, reverse=True)  == ["three", "four", "two"] # longest to shortest

zip​

def zip(*args: typing.Iterable) -> list

zip: zip several iterables together

zip() returns a new list of n-tuples formed from corresponding elements of each of the n iterable sequences provided as arguments to zip. That is, the first tuple contains the first element of each of the sequences, the second element contains the second element of each of the sequences, and so on. The result list is only as long as the shortest of the input sequences.

zip()                           == []
zip(range(5))                   == [(0,), (1,), (2,), (3,), (4,)]
zip(range(5), "abc".elems())    == [(0, "a"), (1, "b"), (2, "c")]