Inspecting Call Stacks¶
In this book, for many purposes, we need to look up a function's location, source code, or simply definition. The class StackInspector
provides a number of convenience methods for this purpose.
Prerequisites
- This is an internal helper class.
- Understanding how frames and local variables are represented in Python helps.
Synopsis¶
To use the code provided in this chapter, write
>>> from debuggingbook.StackInspector import <identifier>
and then make use of the following features.
StackInspector
is typically used as superclass, providing its functionality to subclasses.
Here is an example of how to use caller_function()
. The test()
function invokes an internal method caller()
of StackInspectorDemo
, which in turn invokes callee()
:
Function | Class | |
---|---|---|
callee() |
StackInspectorDemo |
|
caller() |
StackInspectorDemo |
invokes $\uparrow$ |
test() |
(main) | invokes $\uparrow$ |
-/- | (main) | invokes $\uparrow$ |
Using caller_function()
, callee()
determines the first caller outside a StackInspector
class and prints it out – i.e., <function test>
.
>>> class StackInspectorDemo(StackInspector):
>>> def callee(self) -> None:
>>> func = self.caller_function()
>>> assert func.__name__ == 'test'
>>> print(func)
>>>
>>> def caller(self) -> None:
>>> self.callee()
>>> def test() -> None:
>>> demo = StackInspectorDemo()
>>> demo.caller()
>>> test()
<function test at 0x1076188b0>
Here are all methods defined in this chapter:
Inspecting Call Stacks¶
StackInspector
is a class that provides a number of utility functions to inspect a call stack, notably to identify caller functions.
When tracing or instrumenting functions, a common issue is to identify the currently active functions. A typical situation is depicted below, where my_inspector()
currently traces a function called function_under_test()
.
Function | Class | |
---|---|---|
... | StackInspector |
|
caller_frame() |
StackInspector |
invokes $\uparrow$ |
caller_function() |
StackInspector |
invokes $\uparrow$ |
my_inspector() |
some inspector; a subclass of StackInspector |
invokes $\uparrow$ |
function_under_test() |
(any) | is traced by $\uparrow$ |
-/- | (any) | invokes $\uparrow$ |
To determine the calling function, my_inspector()
could check the current frame and retrieve the frame of the caller. However, this caller could be some tracing function again invoking my_inspector()
. Therefore, StackInspector
provides a method caller_function()
that returns the first caller outside a StackInspector
class. This way, a subclass of StackInspector
can define an arbitrary set of functions (and call stack); caller_function()
will always return a function outside the StackInspector
subclass.
import bookutils.setup
from types import FunctionType, FrameType, TracebackType
The method caller_frame()
walks the current call stack from the current frame towards callers (using the f_back
attribute of the current frame) and returns the first frame that is not a method or function from the current StackInspector
class or its subclass. To determine this, the method our_frame()
determines whether the given execution frame refers to one of the methods of StackInspector
or one of its subclasses.
class StackInspector:
"""Provide functions to inspect the stack"""
def caller_frame(self) -> FrameType:
"""Return the frame of the caller."""
# Walk up the call tree until we leave the current class
frame = cast(FrameType, inspect.currentframe())
while self.our_frame(frame):
frame = cast(FrameType, frame.f_back)
return frame
def our_frame(self, frame: FrameType) -> bool:
"""Return true if `frame` is in the current (inspecting) class."""
return isinstance(frame.f_locals.get('self'), self.__class__)
When we access program state or execute functions, we do so in the caller's environment, not ours. The caller_globals()
method acts as replacement for globals()
, using caller_frame()
.
class StackInspector(StackInspector):
def caller_globals(self) -> Dict[str, Any]:
"""Return the globals() environment of the caller."""
return self.caller_frame().f_globals
def caller_locals(self) -> Dict[str, Any]:
"""Return the locals() environment of the caller."""
return self.caller_frame().f_locals
The method caller_location()
returns the caller's function and its location. It does a fair bit of magic to retrieve nested functions, by looking through global and local variables until a match is found. This may be simplified in the future.
Location = Tuple[Callable, int]
class StackInspector(StackInspector):
def caller_location(self) -> Location:
"""Return the location (func, lineno) of the caller."""
return self.caller_function(), self.caller_frame().f_lineno
The function search_frame()
allows searching for an item named name
, walking up the call stack. This is handy when trying to find local functions during tracing, for whom typically only the name is provided.
class StackInspector(StackInspector):
def search_frame(self, name: str, frame: Optional[FrameType] = None) -> \
Tuple[Optional[FrameType], Optional[Callable]]:
"""
Return a pair (`frame`, `item`)
in which the function `name` is defined as `item`.
"""
if frame is None:
frame = self.caller_frame()
while frame:
item = None
if name in frame.f_globals:
item = frame.f_globals[name]
if name in frame.f_locals:
item = frame.f_locals[name]
if item and callable(item):
return frame, item
frame = cast(FrameType, frame.f_back)
return None, None
def search_func(self, name: str, frame: Optional[FrameType] = None) -> \
Optional[Callable]:
"""Search in callers for a definition of the function `name`"""
frame, func = self.search_frame(name, frame)
return func
If we cannot find a function by name, we can create one, using create_function()
.
class StackInspector(StackInspector):
# Avoid generating functions more than once
_generated_function_cache: Dict[Tuple[str, int], Callable] = {}
def create_function(self, frame: FrameType) -> Callable:
"""Create function for given frame"""
name = frame.f_code.co_name
cache_key = (name, frame.f_lineno)
if cache_key in self._generated_function_cache:
return self._generated_function_cache[cache_key]
try:
# Create new function from given code
generated_function = cast(Callable,
FunctionType(frame.f_code,
globals=frame.f_globals,
name=name))
except TypeError:
# Unsuitable code for creating a function
# Last resort: Return some function
generated_function = self.unknown
except Exception as exc:
# Any other exception
warnings.warn(f"Couldn't create function for {name} "
f" ({type(exc).__name__}: {exc})")
generated_function = self.unknown
self._generated_function_cache[cache_key] = generated_function
return generated_function
The method caller_function()
puts all of these together, simply looking up and returning the currently calling function – and creating one if it cannot be found.
class StackInspector(StackInspector):
def caller_function(self) -> Callable:
"""Return the calling function"""
frame = self.caller_frame()
name = frame.f_code.co_name
func = self.search_func(name)
if func:
return func
if not name.startswith('<'):
warnings.warn(f"Couldn't find {name} in caller")
return self.create_function(frame)
def unknown(self) -> None: # Placeholder for unknown functions
pass
The method is_internal_error()
allows us to differentiate whether some exception was raised by StackInspector
(or a subclass) – or whether it was raised by the inspected code.
import traceback
class StackInspector(StackInspector):
def is_internal_error(self, exc_tp: Type,
exc_value: BaseException,
exc_traceback: TracebackType) -> bool:
"""Return True if exception was raised from `StackInspector` or a subclass."""
if not exc_tp:
return False
for frame, lineno in traceback.walk_tb(exc_traceback):
if self.our_frame(frame):
return True
return False
Lessons Learned¶
- In Python, inspecting objects at runtime is easy.
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