einspect 0.4.1.post1

Extended Inspect - view and modify memory structs of runtime objects.

einspect

Extended Inspections for CPython

Documentation

• View and modify memory structures of live objects.
• Able to mutate immutable objects like tuples and ints.
• Modify slot functions or attributes of built-in types.
• Fully typed, extensible framework in pure Python.

Check detailed states of built-in objects

from einspect import view

ls = [1, 2, 3]
v = view(ls)
print(v.info())

PyListObject(at 0x2833738):
   ob_refcnt: Py_ssize_t = 5
   ob_type: *PyTypeObject = &[list]
   ob_item: **PyObject = &[&[1], &[2], &[3]]
   allocated: Py_ssize_t = 4

Mutate tuples, strings, ints, or other immutable types

from einspect import view

t = ("A", "B")
view(t)[1] = 10
print(t)

("A", 10)

from einspect import view

a = "hello"
b = 100

view(a).buffer[:] = b"world"
view(b).value = 5

print("hello", 100)

world 5

Modify attributes of built-in types

from einspect import view

v = view(int)
v["__iter__"] = lambda self: iter(range(self))
v["__str__"] = lambda self: "custom: " + repr(self)

for i in 3:
    print(i)

custom: 0
custom: 1
custom: 2

Move objects in memory

from einspect import view

s = "meaning of life"

v = view(s)
with v.unsafe():
    v <<= 42

print("meaning of life")
print("meaning of life" == 42)

42
True

Fully typed interface

Table of Contents

• Views
  • Using the einspect.view constructor
  • Inspecting struct attributes

Views

Using the einspect.view constructor

This is the recommended and simplest way to create a View onto an object. Equivalent to constructing a specific View subtype from einspect.views, except the choice of subtype is automatic based on object type.

from einspect import view

print(view(1))
print(view("hello"))
print(view([1, 2]))
print(view((1, 2)))

IntView(<PyLongObject at 0x102058920>)
StrView(<PyUnicodeObject at 0x100f12ab0>)
ListView(<PyListObject at 0x10124f800>)
TupleView(<PyTupleObject at 0x100f19a00>)

Inspecting struct attributes

Attributes of the underlying C Struct of objects can be accessed through the view's properties.

from einspect import view

ls = [1, 2]
v = view(ls)

# Inherited from PyObject
print(v.ref_count)  # ob_refcnt
print(v.type)       # ob_type
# Inherited from PyVarObject
print(v.size)       # ob_size
# From PyListObject
print(v.item)       # ob_item
print(v.allocated)  # allocated

4
<class 'tuple'>
3
<einspect.structs.c_long_Array_3 object at 0x105038ed0>

2. Writing to view attributes

Writing to these attributes will affect the underlying object of the view.

Note that most memory-unsafe attribute modifications require entering an unsafe context manager with View.unsafe()

with v.unsafe():
    v.size -= 1

print(obj)

(1, 2)

Since items is an array of integer pointers to python objects, they can be replaced by id() addresses to modify index items in the tuple.

from einspect import view

tup = (100, 200)

with view(tup).unsafe() as v:
    s = "dog"
    v.item[0] = id(s)

print(tup)

('dog', 200)

>> Process finished with exit code 139 (interrupted by signal 11: SIGSEGV)

So here we did set the item at index 0 with our new item, the string "dog", but this also caused a segmentation fault. Note that the act of setting an item in containers like tuples and lists "steals" a reference to the object, even if we only supplied the address pointer.

To make this safe, we will have to manually increment a ref-count before the new item is assigned. To do this we can either create a view of our new item, and increment its ref_count += 1, or use the apis from einspect.api, which are pre-typed implementations of ctypes.pythonapi methods.

from einspect import view
from einspect.api import Py

tup = (100, 200)

with view(tup).unsafe() as v:
    a = "bird"
    Py.IncRef(a)
    v.item[0] = id(a)

    b = "kitten"
    Py.IncRef(b)
    v.item[1] = id(b)

print(tup)

('bird', 'kitten')

🎉 No more seg-faults, and we just successfully set both items in an otherwise immutable tuple.

To make the above routine easier, you can access an abstraction by simply indexing the view.

from einspect import view

tup = ("a", "b", "c")

v = view(tup)
v[0] = 123
v[1] = "hm"
v[2] = "🤔"

print(tup)

(123, 'hm', '🤔')