发布于 2015-08-30 07:59:04 | 169 次阅读 | 评论: 0 | 来源: 网络整理
Most programmers know that Python can evaluate or execute source code provided in the form of a string. For example:
>>> x = 42
>>> eval('2 + 3*4 + x')
56
>>> exec('for i in range(10): print(i)')
0
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9
>>>
However, the ast module can be used to compile Python source code into an abstract syntax tree (AST) that can be analyzed. For example:
>>> import ast
>>> ex = ast.parse('2 + 3*4 + x', mode='eval')
>>> ex
<_ast.Expression object at 0x1007473d0>
>>> ast.dump(ex)
"Expression(body=BinOp(left=BinOp(left=Num(n=2), op=Add(),
right=BinOp(left=Num(n=3), op=Mult(), right=Num(n=4))), op=Add(),
right=Name(id='x', ctx=Load())))"
>>> top = ast.parse('for i in range(10): print(i)', mode='exec')
>>> top
<_ast.Module object at 0x100747390>
>>> ast.dump(top)
"Module(body=[For(target=Name(id='i', ctx=Store()),
iter=Call(func=Name(id='range', ctx=Load()), args=[Num(n=10)],
keywords=[], starargs=None, kwargs=None),
body=[Expr(value=Call(func=Name(id='print', ctx=Load()),
args=[Name(id='i', ctx=Load())], keywords=[], starargs=None,
kwargs=None))], orelse=[])])"
>>>
Analyzing the source tree requires a bit of study on your part, but it consists of a collection of AST nodes. The easiest way to work with these nodes is to define a visitor class that implements various visit_NodeName() methods where NodeName() matches the node of interest. Here is an example of such a class that records information about which names are loaded, stored, and deleted.
import ast
class CodeAnalyzer(ast.NodeVisitor):
def __init__(self):
self.loaded = set()
self.stored = set()
self.deleted = set()
def visit_Name(self, node):
if isinstance(node.ctx, ast.Load):
self.loaded.add(node.id)
elif isinstance(node.ctx, ast.Store):
self.stored.add(node.id)
elif isinstance(node.ctx, ast.Del):
self.deleted.add(node.id)
# Sample usage
if __name__ == '__main__':
# Some Python code
code = '''
for i in range(10):
print(i)
del i
'''
# Parse into an AST
top = ast.parse(code, mode='exec')
# Feed the AST to analyze name usage
c = CodeAnalyzer()
c.visit(top)
print('Loaded:', c.loaded)
print('Stored:', c.stored)
print('Deleted:', c.deleted)
If you run this program, you’ll get output like this:
Loaded: {'i', 'range', 'print'}
Stored: {'i'}
Deleted: {'i'}
Finally, ASTs can be compiled and executed using the compile() function. For example:
>>> exec(compile(top,'<stdin>', 'exec'))
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>>>
The fact that you can analyze source code and get information from it could be the start of writing various code analysis, optimization, or verification tools. For instance, instead of just blindly passing some fragment of code into a function like exec(), you could turn it into an AST first and look at it in some detail to see what it’s doing. You could also write tools that look at the entire source code for a module and perform some sort of static analysis over it.
It should be noted that it is also possible to rewrite the AST to represent new code if you really know what you’re doing. Here is an example of a decorator that lowers globally accessed names into the body of a function by reparsing the function body’s source code, rewriting the AST, and recreating the function’s code object:
# namelower.py
import ast
import inspect
# Node visitor that lowers globally accessed names into
# the function body as local variables.
class NameLower(ast.NodeVisitor):
def __init__(self, lowered_names):
self.lowered_names = lowered_names
def visit_FunctionDef(self, node):
# Compile some assignments to lower the constants
code = '__globals = globals()n'
code += 'n'.join("{0} = __globals['{0}']".format(name)
for name in self.lowered_names)
code_ast = ast.parse(code, mode='exec')
# Inject new statements into the function body
node.body[:0] = code_ast.body
# Save the function object
self.func = node
# Decorator that turns global names into locals
def lower_names(*namelist):
def lower(func):
srclines = inspect.getsource(func).splitlines()
# Skip source lines prior to the @lower_names decorator
for n, line in enumerate(srclines):
if '@lower_names' in line:
break
src = 'n'.join(srclines[n+1:])
# Hack to deal with indented code
if src.startswith((' ','t')):
src = 'if 1:n' + src
top = ast.parse(src, mode='exec')
# Transform the AST
cl = NameLower(namelist)
cl.visit(top)
# Execute the modified AST
temp = {}
exec(compile(top,'','exec'), temp, temp)
# Pull out the modified code object
func.__code__ = temp[func.__name__].__code__
return func
return lower
To use this code, you would write code such as the following:
INCR = 1
@lower_names('INCR')
def countdown(n):
while n > 0:
n -= INCR
The decorator rewrites the source code of the countdown() function to look like this:
def countdown(n):
__globals = globals()
INCR = __globals['INCR']
while n > 0:
n -= INCR
In a performance test, it makes the function run about 20% faster.
Now, should you go applying this decorator to all of your functions? Probably not. However, it’s a good illustration of some very advanced things that might be possible through AST manipulation, source code manipulation, and other techniques.
This recipe was inspired by a similar recipe at ActiveState that worked by manipulating Python’s byte code. Working with the AST is a higher-level approach that might be a bit more straightforward. See the next recipe for more information about byte code.