How to tell if a string contains valid Python code

Question

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.

@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
– user395760, Commented Aug 7, 2012 at 22:07
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
– Joran Beasley, Commented Aug 7, 2012 at 22:09
@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
– user395760, Commented Aug 7, 2012 at 22:10

asmeurer · Accepted Answer · 2012-08-07 22:14:18Z

32

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

edited Aug 7, 2012 at 22:14

asmeurer

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answered Aug 7, 2012 at 22:06

phihag

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2 Comments

asmeurer Over a year ago

I edited out the bare except. I will accept (no pun intended) this answer as soon as the site lets me.

Veedrac Over a year ago

Fails on bare return. Use @leoluk's answer instead.

leoluk · Accepted Answer · 2012-08-07 22:06:55Z

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).

answered Aug 7, 2012 at 22:06

leoluk

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4 Comments

user395760 Over a year ago

compile existed before and is nowhere the full compiler module. ast is closer.

asmeurer Over a year ago

But compile wants filenames and such. I just want to check a string, not read or write to any files.

leoluk Over a year ago

Just pass something like <string> as filename. Full details in the linked docs.

user395760 Over a year ago

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.

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