how to get the line number of an error from exec or execfile in Python

Question

Let's say I have the following multi-line string:

cmd = """
    a = 1 + 1
    b = [
       2 + 2,
       4 + 4,
    ]
    bork bork bork
"""

and I want to execute it in a particular scope:

scope = {}
exec( cmd, scope )
print scope[ 'b' ]

There's a SyntaxError at line 6 of the command, and I want to be able to report that to the user. How do I get the line number? I've tried this:

try:
    exec( cmd, scope )  # <-- let's say this is on line 123 of the source file
except Exception, err:
    a, b, c = sys.exc_info()
    line_number = c.tb_lineno  # <-- this gets me 123,  not 6
    print "%s at line %d (%s)" % ( a, line_number, b.message )

...but I get the line number of the exec statement, not the line number within the multi-line command.

Update: it turns out the handling of the type of exception that I arbitrarily chose for this example, the SyntaxError, is different from the handling of any other type. To clarify, I'm looking a solution that copes with any kind of exception.

Answer 1

For syntax errors, the source line number is available as the lineno flag on the exception object itself, in your case stored in err. This is specific to syntax errors where the line number is an integral part of the error:

>>> cmd = """
... 1 \ +
... 2 * "
... """
>>> try:
...   exec cmd
... except SyntaxError as err:
...   print err.lineno
... 
2

If you want to also handle other errors, add a new except block except Exception, err, and use the traceback module to compute the line number for the runtime error.

import sys
import traceback

class InterpreterError(Exception): pass

def my_exec(cmd, globals=None, locals=None, description='source string'):
    try:
        exec(cmd, globals, locals)
    except SyntaxError as err:
        error_class = err.__class__.__name__
        detail = err.args[0]
        line_number = err.lineno
    except Exception as err:
        error_class = err.__class__.__name__
        detail = err.args[0]
        cl, exc, tb = sys.exc_info()
        line_number = traceback.extract_tb(tb)[-1][1]
    else:
        return
    raise InterpreterError("%s at line %d of %s: %s" % (error_class, line_number, description, detail))

Examples:

>>> my_exec("1+1")  # no exception
>>>
>>> my_exec("1+1\nbork")
...
InterpreterError: NameError at line 2 of source string: name 'bork' is not defined
>>>
>>> my_exec("1+1\nbork bork bork")
...
InterpreterError: SyntaxError at line 2 of source string: invalid syntax
>>>
>>> my_exec("1+1\n'''")
...
InterpreterError: SyntaxError at line 2 of source string: EOF while scanning triple-quoted string