This mini-package lets you get information about what a frame is currently doing, particularly the AST node being executed.
- How does it work?
- Is it reliable?
- Which nodes can it identify?
- Libraries that use this
Getting the AST node
import executing node = executing.Source.executing(frame).node
node will be an AST node (from the
ast standard library module) or None if the node couldn't be identified (which may happen often and should always be checked).
node will always be the same instance for multiple calls with frames at the same point of execution.
Getting the source code of the node
For this you will need to separately install the
asttokens library, then obtain an
or use one of the convenience methods:
__qualname__ of the current function
Everything goes through the
Source class. Only one instance of the class is created for each filename. Subclassing it to add more attributes on creation or methods is recommended. The classmethods such as
executing will respect this. See the source code and docstrings for more detail.
pip install executing
If you don't like that you can just copy the file
executing.py, there are no dependencies (but of course you won't get updates).
How does it work?
Suppose the frame is executing this line:
and in particular it's currently obtaining the attribute
self.foo. Looking at the bytecode, specifically
frame.f_code.co_code[frame.f_lasti], we can tell that it's loading an attribute, but it's not obvious which one. We can narrow down the statement being executed using
frame.f_lineno and find the two
ast.Attribute nodes representing
bar.x. How do we find out which one it is, without recreating the entire compiler in Python?
The trick is to modify the AST slightly for each candidate expression and observe the changes in the bytecode instructions. We change the AST to this:
(self.foo ** 'longuniqueconstant')(bar.x)
and compile it, and the bytecode will be almost the same but there will be two new instructions:
LOAD_CONST 'longuniqueconstant' BINARY_POWER
and just before that will be a
LOAD_ATTR instruction corresponding to
self.foo. Seeing that it's in the same position as the original instruction lets us know we've found our match.
Is it reliable?
Yes - if it identifies a node, you can trust that it's identified the correct one. The tests are very thorough - in addition to unit tests which check various situations directly, there are property tests against a large number of files (see the filenames printed in this build) with real code. Specifically, for each file, the tests:
- Identify as many nodes as possible from all the bytecode instructions in the file, and assert that they are all distinct
- Find all the nodes that should be identifiable, and assert that they were indeed identified somewhere
In other words, it shows that there is a one-to-one mapping between the nodes and the instructions that can be handled. This leaves very little room for a bug to creep in.
executing checks that the instructions compiled from the modified AST exactly match the original code save for a few small known exceptions. This accounts for all the quirks and optimisations in the interpreter.
Which nodes can it identify?
Currently it works in almost all cases for the following
x + y(doesn't include
notbut only works sometimes)
a < b(not for chains such as
0 < p < 1)
The plan is to extend to more operations in the future.
Libraries that use this
snoop: A feature-rich and convenient debugging library. Uses
executingto show the operation which caused an exception and to allow the
ppfunction to display the source of its arguments.
heartrate: A simple real time visualisation of the execution of a Python program. Uses
executingto highlight currently executing operations, particularly in each frame of the stack trace.
sorcery: Dark magic delights in Python. Uses
executingto let special callables called spells know where they're being called from.