Having problems with ANTLR4 in Python and getting an ErrorListener to work - antlr4

Question

I am declaring my error listener like this:
class GeneratorErrorListener(ErrorListener):
def __init__(self, listener):
super().__init__()
self.listener = listener
def systaxError(self, recognizer, offendingSymbol, line, col, msg, e):
log_it("Syntax error at line {} col {}: {}".format(line, col, msg))
I am not yet making use of the listener passed in, but will when I get it working.
and setting it up like this:
...
# Set up new error listener
parser.removeErrorListeners()
parser.addErrorListener(GeneratorErrorListener(listener))
tree = parser.protocol()
...
walker.walk(listener, tree)
Then I am testing it with some input that has a syntax error (AFAICS):
The grammar fragment is:
enumEltDecl : INT '=' ID ( ':' STRING)?
| 'default' '=' STRING
;
enumDecl: 'enum' ID ( ':' ID )? '{' enumEltDecl (',' enumEltDecl )*
(',')? '}' ;
and I can parse those things fine. However, the following input which I think should be a syntax error, and does cause parsing to stop, does not invoke the error listener:
emum some_emum:uint8 {
};
It should have at least one enumEltDecl.
Any thoughts on what I have done wrong? I have looked at the runtime code for the ErrorListener class and it seems straightforward.
More Information
The code is here: https://gitlab.com/realrichardsharpe/wireshark-generator-python
Use the following steps to see the issue:
cd src
./GenTool.py -t C ../test-data/syntax-error.proto
You will see the following output:
#include "config.h"
#include <epan/packet.h>
#include <epan/expert.h>
//Generating code for enum cmd_enum
enum cmd_enum {
CMD1 = 0x14;
CMD2 = 0x15;
CMD3 = 0x28;
CMD4 = 0x29;
CMD5 = 0x3C;
CMD5 = 0x3D;
};
//We have a uint8
static const range_string cmd_enum_rvals[] = {
{ 0, 19, "Reserved", }
{ 0x14, 0x14, "cmd1" },
{ 0x15, 0x15, "cmd2" },
{ 22, 39, "Reserved", }
{ 0x28, 0x28, "cmd3" },
{ 0x29, 0x29, "cmd4" },
{ 42, 59, "Reserved", }
{ 0x3C, 0x3C, "cmd5" },
{ 0x3D, 0x3D, "cmd6" },
{ 62. 255, "Reserved" },
};
And it stops without my ErrorListener being called. The ErrorListener is in GenTool.py.

Answer 1

Strangely, with a little rearrangement of the code and after lots of debugging it now seems to be working because I get the following errors with a different set of input data:
./GenTool.py -t C xxx.proto
line 3:0 mismatched input '}' expecting {'default', INT}
line 4:0 missing ';' at '<EOF>'
Syntax error on line 1
The first two lines are generated by my error listener.
UPDATE: With a little comparison, I discovered that my test case had a symbol in it that is not recognized by the grammar and things went off the rails at that point.
The real problem was that my grammar was incorrect. The first line should have been:
protocol : protoDecl+ EOF ;
In my original grammer the EOF was missing which caused the parser to stop when it hit something that did not match the grammer.