As discussed elsewhere, pytype’s primary model for function and method calls is the function signature. A function is modelled as
def f(arg1: type1, ...) -> return_type: mutated_arg = new_type ...
and when the pytype VM calls the function, it matches the argument types against the types of the passed-in variables, performs any type mutations, and pushes a return type onto the stack.
Since the VM deals with types rather than values, we typically do not care about any side effects a function may have (other than the aforementioned type mutations). However, there are some functions that either have complex type-level side effects (e.g. adding class attributes via metaprogramming) or have type effects that depend on the values of the arguments. Type matching and unification are insufficient to cover this; we need to directly manipulate the abstract values we use to represent python objects.
There are two main sections of the pytype codebase dealing with these functions
super()that are part of the python core language, and overlays, which handle both standard and third-party libraries.
NOTE: These special functions still need type signatures to interoperate with
the rest of pytype; in the case of
special_builtins.py the corresponding
signatures can be found in
special_builtins.py defines two classes,
which all special builtins inherit from. These are subclasses of
The main use case for these builtins is to override the
call method, following
python’s implementation of them as callable functions/classes. For example,
next(x) compiles to
LOAD_GLOBAL 0 (next) LOAD_FAST 0 (x) CALL_FUNCTION 1
that is, it loads
next as a global object and calls its
call method. When
pytype analyses this bytecode it needs to implement any special behaviour when
next is called, not when it is loaded.
The return types of special builtins are sometimes special objects too; we model
these with corresponding custom classes deriving directly from
abstract.BaseValue and implementing the right behaviour for various
attribute and method accesses.
VirtualMachine.__init__ defines a mapping
self.special_builtins from the
names of python builtins to instances of the corresponding special builtins.
Each special builtin then provides its own implementation of
call, and when
the VM encounters a call to e.g.
next(arg) it delegates to
The VM loads python builtins by calling
load_builtin() in turn calls
load_special_builtin() to check if the name we
are loading is defined in the
self.special_builtins mapping mentioned above.
While each special builtin is ultimately custom code mirroring the details of the corresponding python builtin, there are some common techniques that all the implementations use.
A lot of special functions delegate to a method call on their first argument,
x.__abs__() internally. The base
get_underlying_method helper for subclasses to use; e.g.
self.get_underlying_method(node, arg, "__abs__")
and then just reinvokes the regular
vm.call_function() but now calling the
x.__abs__ rather than the built in function
abs (this is a good
example of something that is fairly straightforward but nevertheless impossible
to do via type signatures).
Slots are a python
mechanism by which a
class can provide a single
__slots__ dictionary for attribute lookup, rather
than a per-instance
__dict__. Pytype uses an analogous mechanism internally by
which a class can provide slots to support custom method overrides.
Special builtins that need to override methods other than
call mix in
mixin.HasSlots and provide a list of slots and an overridden implementation
for each one. As with
call these implementations need to do something more
complex than the default of matching a signature and providing the correct
special_builtins.PropertyInstance binds the
__get__ method of
the property to a decorated method in the target code by setting a slot:
class PropertyInstance(mixin.HasSlots, ...): def __init__(self, fget, ...): # sets the InterpreterFunction to call self.fget = fget # will be invoked when the target bytecode calls the property self.set_slot("__get__", self.fget_slot) def fget_slot(self, ...): return self.vm.call_function(self.fget, ...)
NOTE: Slots are implemented via the
get_special_attribute method in
abstract.py/BaseValue hierarchy and the corresponding override
As mentioned earlier, calling
some_builtin.call() often returns another
special object. This is typically the case for a builtin exhibiting multi-stage
behaviour (e.g. a method decorator has two relevant invocation points, first
wrapping a method and returning a new method, and then performing some custom
behaviour when the wrapped method is called).
We will take a look at the implementation of
@staticmethod as an example.
class A: @staticmethod def f(cls): pass
LOAD_NAME 3 (staticmethod) LOAD_CONST 1 (<code object f>) LOAD_CONST 2 ('A.f') MAKE_FUNCTION 0 CALL_FUNCTION 1 STORE_NAME 4 (f)
i.e. it loads
staticmethod, loads the code object for
staticmethod(f) and stores the return value as
f again; this is our wrapped
A.f() will now find the wrapped method; i.e. we have (at
runtime!) taken the instance method
A().f and moved it into the static method
Pytype replicates this behaviour by providing a
StaticMethod class, whose
call method takes in a function (specifically a variable whose binding is an
abstract.InterpreterFunction object), and returns a
that wraps the original variable.
StaticMethodInstance in turn wraps the
underlying function and provides an object whose
get_class() method returns
special_builtins.StaticMethod and whose
__get__ slot returns the original
function. (The details of
StaticMethodInstance don’t matter too much for now,
but note the two-stage process by which we have achieved the desired method
overrides on the
As a side note, when reading the
special_builtins code it is essential to keep
clear the distinction between Data (representation of python objects) and
Variables (typegraph representations of a python variable, potentially with
multiple Bindings to data). The special builtins’
call() methods all take
Variable form, perform computations on the underlying
BaseValues, and then construct a new
Variable with the results of
Look for the pattern
def call(*args): result = self.vm.program.NewVariable() # unpack data from args ... # special builtin code here ... # update result variable result.AddBinding(some_data) # and/or result.PasteVariable(some_variable) return node, result