How to check python code's syntax error before running it

Question

I am developing a tool which has to accept a file as an input, check syntax errors, compile it and do something after that.

For example, I have a file run.py:

a=5
b=c
print b

This should clearly show a syntax error while compiling because 'c' is not defined

I tried to use

try:
    py_compile.compile("source_program/run.py", doraise=True)
    print "Compiled"
except:
    print "Error while compiling"

I get the output "Compiled" instead of "Error while compiling"

If I modify the run.py file as:

a=5
b=c/  #Instead of b=c
print b

Then I get the output "Error while compiling"

What don't I get an error message in the first case?

b = c with c not defined is not a SyntaxError but a NameError — joaquin
– joaquin, Commented Feb 13, 2014 at 7:39

user2357112 · Accepted Answer · 2014-02-13 07:55:23Z

9

It's not a syntax error. b=c is perfectly valid syntax, whether or not c exists. In fact, some other module could have done

import __builtin__
__builtin__.c = 3

in which case there would be a built-in c variable with value 3 available to all modules, and your code would run fine.

For a somewhat less pathological example, if the file contains a * import such as

from numpy import *

the import will dump a whole bunch of names into the module's global namespace, and there's no way to tell what those names are. Even without import *, though, Python can't be sure that a reference to an unknown name is an error at compile time.

If you want to detect semantic errors such as this, you'll need a more complex analysis of the program. Integrating with an existing linter like pylint, as suggested by NPE, is likely to be more productive than writing your own tool. If you really want to do it yourself, you can parse the code with ast.parse and examine the AST, going statement by statement to see what variables exist at what points. You'll still never catch all bugs, but you'll find quite a few.

edited Feb 13, 2014 at 7:55

answered Feb 13, 2014 at 7:37

user2357112

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3 Comments

mea Over a year ago

Thank you for the reply. But I don't want it to run fine. I want it to show an error. My tool should be able to detect an error because 'c' is not defined.

mea Over a year ago

If I run the program run.py, i get an error message on the console "NameError: name 'c' is not defined" So when I run my tool, it should do something similar

user2357112 Over a year ago

@Melvin: Answer expanded. Note that while NameErrors can be detected pretty reliably, it's not perfect, and you'll have little luck detecting things that need more runtime information, like IndexErrors.

NPE · Accepted Answer · 2014-02-13 07:38:20Z

4

It's a tricky one, for many reasons.

It might not be a bad idea to try and integrate with pylint instead of trying to come up with your own.

answered Feb 13, 2014 at 7:38

NPE

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Comments

gregkow · Accepted Answer · 2014-02-13 07:38:24Z

2

c not being defined actually isn't a compile-time error. Python, when run, only runs into problems with undefined variables during runtime. This is not something that would be caught by any Python compiler.

answered Feb 13, 2014 at 7:38

gregkow

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