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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.

Stefano Borini
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asmeurer
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    @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. –  Aug 07 '12 at 22:07
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    @delnan waiting for gcc-python now – Stefano Borini Aug 07 '12 at 22:08
  • 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 Aug 07 '12 at 22:09
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    @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. –  Aug 07 '12 at 22:10
  • i think pylint would catch it (in most cases) ... – Joran Beasley Aug 07 '12 at 22:11
  • @JoranBeasley Yeah, because the example error you gave is trivial. PyLint and co. are okay, but won't catch much a quick glance won't see either. I'm not aware of any tool complaining about calls like `f(MyClass())` where `def f(x): return 1 + x` in another module, but not about `f(MyClassWith__radd__())`. –  Aug 07 '12 at 22:11
  • Only syntax errors. And it needs to be part of the standard library. – asmeurer Aug 07 '12 at 22:12

2 Answers2

25

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
asmeurer
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phihag
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4

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).

leoluk
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    `compile` existed before and is nowhere the full `compiler` module. `ast` is closer. –  Aug 07 '12 at 22:08
  • But `compile` wants filenames and such. I just want to check a string, not read or write to any files. – asmeurer Aug 07 '12 at 22:13
  • Just pass something like `` as filename. Full details in the linked docs. – leoluk Aug 07 '12 at 22:15
  • Aside from that, the `compile` function has a few quirks (requires specific newline characters, and raises a TypeError out of friggin' nowhere when there are NUL bytes in the input) other options don't have. –  Aug 07 '12 at 22:19