Return location of MEL script using maya whatIs plus directly print the source code of the procedure

maya_what_is_mel_code.py

from maya import cmds
import re
import os


def get_mel_script_code(name):
    """Return source code of the MEL procedure loaded from a .mel script file"""
    result = mel.eval('whatIs("%s")' % name)
    
    if result == "Unknown":
        raise RuntimeError("Not an existing command: %s" % name)
    
    if result in {"Command", "Run Time Command"}:
        raise RuntimeError("No source for %s: %s" % (result, name))
    
    path = result.split('in:')[-1].strip()
    assert os.path.exists(path), "File not found: %s" % path
    
    # Regex to match start of procedure
    re_proc = re.compile("proc\s+{}\s*\(".format(name))
    
    # Collect source code
    depth = 0
    matched = False
    code = []
    with open(path) as f:
        for line in f.readlines():
            
            # Search for start of procedure
            if not matched and re_proc.search(line):
                code.append(line)
                depth += line.count("{")
                matched = True
                continue
            
            # Once found, process procedure until end
            if matched:
                depth += line.count("{")
                if depth > 0:
                    code.append(line)
                depth -= line.count("}")
                if depth <= 0 and len(code) != 1:
                    # Remove the new line from the end
                    # of the code
                    code[-1] = code[-1].rstrip()
                    break
                    
    if not code:
        # This would be a bug
        raise RuntimError("Source code not found for: %s" % name)
              
    return "".join(code)


def what_is(name, print_procedure=True):
    """Print whatIs info with source code of procedure if from .mel script"""
    
    # Print path
    print(mel.eval('whatIs("%s")' % name))
    
    # Print source code
    try:
        code = get_mel_script_code(name)
        print("Source Code:\n```mel")
        print(code)
        print("```")
    except RuntimeError as exc:
        pass
    
    
# Examples usage:
what_is("expandPolyGroupSelection")
what_is("invertSelection")

Example usage in Python:

what_is("expandPolyGroupSelection")

Result:

// Result: Mel procedure found in: C:/Program Files/Autodesk/Maya2020/scripts/others/expandPolyGroupSelection.mel // 
Source Code:
```mel
global proc expandPolyGroupSelection()
{
	// We need to expand selection to include polygons(12) vertices(31) edges(32) faces(34) UVs(35)
	if( `selectMode -q -o` )
		select `filterExpand -sm 12 -sm 31 -sm 32 -sm 34 -sm 35 -ex false`;
}
```

Or if you are even lazier and have Notepad++ installed. Directly open the source .mel file at the correct line for that function:

import subprocess
import os
import re
from maya import cmds, mel

NOTEPAD_EXE = r"C:\Program Files\Notepad++\notepad++.exe"

def open_mel_source_in_notepad_plusplus(name):
    """Return source code of the MEL procedure loaded from a .mel script file"""
    result = mel.eval('whatIs("%s")' % name)
    
    if result == "Unknown":
        raise RuntimeError("Not an existing command: %s" % name)
    
    if result in {"Command", "Run Time Command"}:
        raise RuntimeError("No source for %s: %s" % (result, name))
    
    path = result.split('in:')[-1].strip()
    assert os.path.exists(path), "File not found: %s" % path
    
    # Regex to match start of procedure
    re_proc = re.compile("proc\s+{}\s*\(".format(name))
    
    line_number = 0
    with open(path) as f:
        for index, line in enumerate(f.readlines()):        
            # Search for start of procedure
            if re_proc.search(line):
                line_number = index
                break
    
    subprocess.Popen([NOTEPAD_EXE, path, "-n{}".format(line_number)])

# Example usage:
open_mel_source_in_notepad_plusplus("invertSelection")