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230 lines
6.1 KiB
230 lines
6.1 KiB
Metadata-Version: 2.1
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Name: pure-eval
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Version: 0.2.2
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Summary: Safely evaluate AST nodes without side effects
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Home-page: http://github.com/alexmojaki/pure_eval
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Author: Alex Hall
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Author-email: alex.mojaki@gmail.com
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License: MIT
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Platform: UNKNOWN
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Classifier: Intended Audience :: Developers
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Classifier: Programming Language :: Python :: 3.5
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Classifier: Programming Language :: Python :: 3.6
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Classifier: Programming Language :: Python :: 3.7
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Classifier: Programming Language :: Python :: 3.8
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Classifier: Programming Language :: Python :: 3.9
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Classifier: Programming Language :: Python :: 3.10
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Classifier: License :: OSI Approved :: MIT License
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Classifier: Operating System :: OS Independent
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Description-Content-Type: text/markdown
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License-File: LICENSE.txt
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Provides-Extra: tests
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Requires-Dist: pytest ; extra == 'tests'
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# `pure_eval`
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[](https://travis-ci.org/alexmojaki/pure_eval) [](https://coveralls.io/github/alexmojaki/pure_eval?branch=master) [](https://pypi.python.org/pypi/pure_eval)
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This is a Python package that lets you safely evaluate certain AST nodes without triggering arbitrary code that may have unwanted side effects.
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It can be installed from PyPI:
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pip install pure_eval
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To demonstrate usage, suppose we have an object defined as follows:
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```python
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class Rectangle:
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def __init__(self, width, height):
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self.width = width
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self.height = height
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@property
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def area(self):
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print("Calculating area...")
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return self.width * self.height
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rect = Rectangle(3, 5)
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```
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Given the `rect` object, we want to evaluate whatever expressions we can in this source code:
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```python
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source = "(rect.width, rect.height, rect.area)"
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```
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This library works with the AST, so let's parse the source code and peek inside:
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```python
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import ast
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tree = ast.parse(source)
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the_tuple = tree.body[0].value
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for node in the_tuple.elts:
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print(ast.dump(node))
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```
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Output:
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```python
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Attribute(value=Name(id='rect', ctx=Load()), attr='width', ctx=Load())
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Attribute(value=Name(id='rect', ctx=Load()), attr='height', ctx=Load())
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Attribute(value=Name(id='rect', ctx=Load()), attr='area', ctx=Load())
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```
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Now to actually use the library. First construct an Evaluator:
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```python
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from pure_eval import Evaluator
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evaluator = Evaluator({"rect": rect})
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```
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The argument to `Evaluator` should be a mapping from variable names to their values. Or if you have access to the stack frame where `rect` is defined, you can instead use:
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```python
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evaluator = Evaluator.from_frame(frame)
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```
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Now to evaluate some nodes, using `evaluator[node]`:
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```python
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print("rect.width:", evaluator[the_tuple.elts[0]])
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print("rect:", evaluator[the_tuple.elts[0].value])
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```
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Output:
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```
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rect.width: 3
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rect: <__main__.Rectangle object at 0x105b0dd30>
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```
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OK, but you could have done the same thing with `eval`. The useful part is that it will refuse to evaluate the property `rect.area` because that would trigger unknown code. If we try, it'll raise a `CannotEval` exception.
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```python
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from pure_eval import CannotEval
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try:
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print("rect.area:", evaluator[the_tuple.elts[2]]) # fails
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except CannotEval as e:
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print(e) # prints CannotEval
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```
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To find all the expressions that can be evaluated in a tree:
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```python
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for node, value in evaluator.find_expressions(tree):
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print(ast.dump(node), value)
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```
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Output:
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```python
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Attribute(value=Name(id='rect', ctx=Load()), attr='width', ctx=Load()) 3
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Attribute(value=Name(id='rect', ctx=Load()), attr='height', ctx=Load()) 5
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Name(id='rect', ctx=Load()) <__main__.Rectangle object at 0x105568d30>
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Name(id='rect', ctx=Load()) <__main__.Rectangle object at 0x105568d30>
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Name(id='rect', ctx=Load()) <__main__.Rectangle object at 0x105568d30>
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```
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Note that this includes `rect` three times, once for each appearance in the source code. Since all these nodes are equivalent, we can group them together:
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```python
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from pure_eval import group_expressions
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for nodes, values in group_expressions(evaluator.find_expressions(tree)):
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print(len(nodes), "nodes with value:", values)
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```
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Output:
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```
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1 nodes with value: 3
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1 nodes with value: 5
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3 nodes with value: <__main__.Rectangle object at 0x10d374d30>
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```
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If we want to list all the expressions in a tree, we may want to filter out certain expressions whose values are obvious. For example, suppose we have a function `foo`:
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```python
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def foo():
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pass
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```
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If we refer to `foo` by its name as usual, then that's not interesting:
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```python
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from pure_eval import is_expression_interesting
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node = ast.parse('foo').body[0].value
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print(ast.dump(node))
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print(is_expression_interesting(node, foo))
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```
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Output:
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```python
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Name(id='foo', ctx=Load())
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False
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```
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But if we refer to it by a different name, then it's interesting:
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```python
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node = ast.parse('bar').body[0].value
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print(ast.dump(node))
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print(is_expression_interesting(node, foo))
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```
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Output:
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```python
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Name(id='bar', ctx=Load())
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True
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```
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In general `is_expression_interesting` returns False for the following values:
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- Literals (e.g. `123`, `'abc'`, `[1, 2, 3]`, `{'a': (), 'b': ([1, 2], [3])}`)
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- Variables or attributes whose name is equal to the value's `__name__`, such as `foo` above or `self.foo` if it was a method.
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- Builtins (e.g. `len`) referred to by their usual name.
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To make things easier, you can combine finding expressions, grouping them, and filtering out the obvious ones with:
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```python
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evaluator.interesting_expressions_grouped(root)
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```
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To get the source code of an AST node, I recommend [asttokens](https://github.com/gristlabs/asttokens).
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Here's a complete example that brings it all together:
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```python
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from asttokens import ASTTokens
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from pure_eval import Evaluator
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source = """
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x = 1
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d = {x: 2}
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y = d[x]
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"""
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names = {}
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exec(source, names)
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atok = ASTTokens(source, parse=True)
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for nodes, value in Evaluator(names).interesting_expressions_grouped(atok.tree):
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print(atok.get_text(nodes[0]), "=", value)
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```
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Output:
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```python
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x = 1
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d = {1: 2}
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y = 2
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d[x] = 2
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```
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