Python low-level AOP using AST rewriting – part I

This post and the next post will address AOP in Python. In general AOP in Python is very simple thanks to Python’s decorators. The aspects which we would like to apply in this post are low-level, meaning they’ll be applied on in-body instructions and not just on method level. The way in which we’re going to implement it will be using code weaving and rewriting.
I previously blogged about similar concept in .Net using Mono Cecil, where we tracked IL instructions.

The topic will be covered by two posts, where the first one will address rewriting code and the second one will deal with replacing the original code.



The general motivation for AOP is to separate the business logic from other functional logic, like logging, security or error handling. Most of the common examples fit the pattern of wrapping the function with new one. Then, perform logic before/after the method is executed. This is very useful, yet, limits our ability to change behavior of specific instructions inside the method which are relevant to the aspect.


During the post we will use a concrete simple example. Let us observe the following example (Python 2.7):

def foo(x):
    return x < 1

print foo(None)

As you probably know, this will print:


This is a common Python (2.7) behavior but might not be intuitive. In general, assuming we had many variables and many comparisons, we’d like to change all to the pattern: VAR is not None and VAR < CONST

The goal of our process will be to transform the method to:

def foo(x):
    return x is not None and x &lt; 1

Where the aspect we’re applying is Update Comparison of None and Constants.

The required steps

The steps required by this solution are the following:

  1. Decorate the method – create an entry point for the mechanism which’ll apply the aspect.
  2. Create an AST from the method –  prepare a modifiable syntax tree from the original method.
  3. Rewrite the AST – find the instructions influenced by the aspect and modify them.
  4. Create bytecode – create identical code to the original one other than newly generated bytecode.
  5. Update the method – replace the original method code with the new one.

Decorating the method

Like the common approach, we will use a decorator to modify the function. We will start from this simple decorator and build over it:

def rewrite_comparisons(original_function):
    assert isinstance(original_function, FunctionType)
    return original_function

def foo(x):
    return x &lt; 1

This decorator does nothing, so far.

Getting code from function

The first challenge is getting the method code from a function and make it modifiable. Since Python provides bytecode by default for a method, we will use built-in inspect to extract the original source code:

function_source_code = inspect.getsource(original_function)

Inspect uses the code locations linked to the function and read them from the source file. The return value is a string with function. This is different from disassembling code from the method bytecode.

We can assume that for our functions the source code is available. Otherwise, this first step will fail, and the processing will need to be in bytecode level (which might be covered in other post). In addition, this constrains us to ensure decorator is called before any other decorator. Otherwise, previous decorators might be ignore since their effect is not reflected in the original source code.

Building an AST (abstract syntax tree)

After the previous line of code extracted the source, we can parse it to an AST. The motivation for building an abstract syntax tree, is that it’s modifiable and we can compile is back to bytecode.

function_source_code = inspect.getsource(original_function)
node = ast.parse(function_source_code)

The node we get is the root one of the parsed code. It links to all the elements in the hierarchy and represents a simplified module code.

Taking for example the foo function, the tree is:

  # the method declaration (foo)
    # the arguments list (x)
    # return instruction
      # comparison of two elements
        # load variable (x)
        # comparison operator (<)
        # load constant (1)

The AST represents the function, while the decorator is omitted for simplicity. As can easily be seen, the tree represents all the content of the method, including declaration, other methods in context if there are and more. Given the AST, we’d like to modify it a fit the need that our aspect requires.

Transforming the AST

AST visitors

We will use the AST visitors as an introduction to syntax tree traversal. The node visitors follow a convention where callback names are of pattern visit_NODETYPE(self, node), where node type can be any these. For example, if we want a callback on method calls, we can define one for the Call node and name it visit_Call(self, node).

In our example, we can visit the compare nodes, and print all the operands:

from ast import NodeVisitor

class ComparisonVisitor(NodeVisitor):
    def visit_Compare(self, node):
        operands = [node.left] + node.comparators
        print '; '.join(type(operand).__name__ for operand in operands)

For every callback, we are assured the type of the node fits the Compare node type. Given the type, we can investigate it’s members. Comparison in Python is composed of operators (one or more) and operands (two or more). In the case of Compare node, the first operand is called left, and the rest are called comparators. One of the reason for the complicated structure is to support expressions like:

0 &lt; x &lt; 100

Using the visitor we can query the nodes, but not modify them. If we visit the the original foo function:

node = ast.parse(inspect.getsource(foo))

The result we expect is:

Name; Num

Since comparison is x < 1, where x is Name load in the context and 1 is a Constant Number in the context.

AST transformers

Python provides transformers, which are a special type of AST visitors. The transformers, in contrast to nodes visitors,  modify the nodes they visit. In our example, we’ll look for nodes that represent comparison between variables and numbers, and then extend them to comply with the aspect.

from ast import NodeTransformer
from _ast import BoolOp, And, Name, Compare, IsNot, Load, Num

class ComparisonTransformer(NodeTransformer):
    def visit_Compare(self, node):
        parts = [node.left] + node.comparators

        # check if any constant number is involved in the comparison
        if not any(isinstance(part, Num) for part in parts):
            return node

        # get all the "variables" involved in the comparison
        names = [element for element in parts if isinstance(element, Name)]
        if len(names) == 0:
            return node

        # create a reference to None
        none = Name(id='None', ctx=Load())
        # create for each variable a node that represents 'var is not None'
        node_verifiers = [Compare(left=name, ops=[IsNot()], comparators=[none]) for name in names]
        # combine the None checks with the original comparison
        # e.g. 'a &lt; b &lt; 1' --&gt; 'a is not None and b is not None and a &lt; b &lt; 1
        return BoolOp(op=And(), values=node_verifiers + [node])

This chunk of code is a simplified (relaxed type input checks no attempts to code location fixes) version of a transformer that visits all nodes of type Compare. The transformer methods names use the same convention as the visitors.

According to the original behavior a new node is being built. This node is a new Boolean expression, which requires all the variables[2] in use to be not None and to satisfy the original comparison.

If we’d look at the output, the the AST will be modified and verify variables are not None before they’re compared to None. The out tree for the modified foo is:

  # the method declaration (foo)
    # the arguments list (x)
    # return instruction
      # the bool expression that combines with And:
      # 1. the original comparison
      # 2. the new check 'VAR is not None'
        # the 'x is not None' comparison
        # the original comparison 'x < 1'

Prepare the node forrecompilation

In the next phase, we’re going to import the new code as temporary module, which will case the declaration of the new method to be executed again. In order to do so, we’d like to remove the rewriter decorator, since we don’t want it to process the modified function. In addition, we rename the function for safety to avoid collisions between the declared function and other locals.  Lastly, we ask python to fix code locations for the new nodes so they can be compiled later on. This is done using fix_missing_locations.

from ast fix_missing_locations

def rewrite_method(node):
    # assuming the method has single decorator (which is the rewriter) - remove it
    # we rename the method to ensure separation from the original one.
    # this step has no real meaning and not really required.
    node.body[0].name = 'internal_method'
    # transform Compare nodes to fit the 'is not None' requirement
    # let python try and fill code locations for the new elements


During the first phase we got as an input a function (through a decorator), then modified it’s body by visiting it’s body using a syntactic level. Lastly, we modified it’s declaration and source locations so it can be safely imported as a new function.

As you probably notice, the only part in this code which is concerned by the aspect is the transformer. Meaning, if we’d like to apply a different aspect the only part which’ll change is the transformer. In our example the ComparisonTransformer is hard-coded for simplicity, but in real solution we’d provide it as an argument to the decorator.

Next phase

In the next phase we’ll use the modified function to generate replacement bytecode.


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