syntax - How to tell if a string contains valid Python code

19
votes

If I have a string of Python code, how do I tell if it is valid, i.e., if entered at the Python prompt, it would raise a SyntaxError or not? I thought that using compiler.parse would work, but apparently that module has been removed in Python 3. Is there a way to do it that also works in Python 3. Obviously, I don't want to execute the code, just check its syntax.

2
pythonsyntaxpython-3.x
@goldenparrot Last time I checked, gcc did not have a Python frontend and static compilers in general sucked at doing dynamic languages, if they even tried. - user395760
@delnan waiting for gcc-python now - Stefano Borini
are you really only looking for syntax errors? or also logical errors( ie g==5 without setting a value for g) as g==5 will ast.parse even with no g anywhere - Joran Beasley
@JoranBeasley Given that such errors are impossible to detect in the general case, research-level hard in many other cases, and still pretty darn expensive to find in all but the most trivial cases, I hope not for his sake. - user395760
i think pylint would catch it (in most cases) ... - Joran Beasley

2 Answers

23
votes

Use ast.parse:

import ast
def is_valid_python(code):
   try:
       ast.parse(code)
   except SyntaxError:
       return False
   return True


>>> is_valid_python('1 // 2')
True
>>> is_valid_python('1 /// 2')
False
3
votes

The compiler module is now a built-in.

compile(source, filename, mode[, flags[, dont_inherit]])

Compile the source into a code or AST object. Code objects can be executed by an exec statement or evaluated by a call to eval(). source can either be a string or an AST object. Refer to the ast module documentation for information on how to work with AST objects.

The AST parser is now a seperate module.

ast.parse(expr, filename='<unknown>', mode='exec')

Parse an expression into an AST node. Equivalent to compile(expr, filename, mode, ast.PyCF_ONLY_AST).