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