Allow the user to specify debugpy path via the environment

src/debugpy/__main__.py

@@ -3,34 +3,67 @@
 # for license information.
 
 import sys
-
+import os
 
 if __name__ == "__main__":
-    # debugpy can also be invoked directly rather than via -m. In this case, the first
-    # entry on sys.path is the one added automatically by Python for the directory
-    # containing this file. This means that import debugpy will not work, since we need
-    # the parent directory of debugpy/ to be in sys.path, rather than debugpy/ itself.
+
+    # There are three ways to run debugpy:
     #
-    # The other issue is that many other absolute imports will break, because they
-    # will be resolved relative to debugpy/ - e.g. `import debugger` will then try
+    # 1. Installed as a module in the current environment (python -m debugpy ...)
+    # 2. Run as a script from source code (python <repo_root>/src/debugpy ...)
+    # 3. Installed as a module in a random directory
+    #
+    # -----
+    #
+    # In the first case, no extra work is needed. Importing debugpy will work as expected.
+    # Also, running 'debugpy' instead of 'python -m debugpy' will work because of the entry point
+    # defined in setup.py.
+    #
+    # -----
+    #
+    # In the second case, sys.path[0] is the one added automatically by Python for the directory 
+    # containing this file. 'import debugpy' will not work since we need the parent directory 
+    # of debugpy/ to be in sys.path, rather than debugpy/ itself. So we need to modify sys.path[0].
+    # Running 'debugpy' will not work because the entry point is not defined in this case.
+    #
+    # -----
+    #
+    # In the third case, running 'python -m debugpy' will not work because the module is not installed
+    # in any environment. Running 'python <path_to_debugpy>' will work, just like the second case. 
+    # But running 'debugpy' will not work because even though the entry point is defined, 
+    # that path is not in sys.path, so 'import debugpy' will fail. So just like in the second case, 
+    # we need to modify sys.path[0].
+    #
+    # -----
+    #
+    # If we modify sys.path, 'import debugpy' will work, but it will break other imports
+    # because they will be resolved relative to debugpy/ - e.g. `import debugger` will try
     # to import debugpy/debugger.py.
     #
-    # To fix both, we need to replace the automatically added entry such that it points
-    # at parent directory of debugpy/ instead of debugpy/ itself, import debugpy with that
-    # in sys.path, and then remove the first entry entry altogether, so that it doesn't
-    # affect any further imports we might do. For example, suppose the user did:
+    # To fix both problems, we need to do the following steps:
+    # 1. Modify sys.path[0] to point at the parent directory of debugpy/ instead of debugpy/ itself.
+    # 2. Import debugpy.
+    # 3. Remove sys.path[0] so that it doesn't affect future imports. 
+    # 
+    # For example, suppose the user did:
     #
     #   python /foo/bar/debugpy ...
     #
-    # At the beginning of this script, sys.path will contain "/foo/bar/debugpy" as the
-    # first entry. What we want is to replace it with "/foo/bar', then import debugpy
-    # with that in effect, and then remove the replaced entry before any more
-    # code runs. The imported debugpy module will remain in sys.modules, and thus all
-    # future imports of it or its submodules will resolve accordingly.
+    # At the beginning of this script, sys.path[0] will contain "/foo/bar/debugpy".
+    # We want to replace it with "/foo/bar', then 'import debugpy', then remove the replaced entry.
+    # The imported debugpy module will remain in sys.modules, and thus all future imports of it 
+    # or its submodules will resolve accordingly.
     if "debugpy" not in sys.modules:
-        # Do not use dirname() to walk up - this can be a relative path, e.g. ".".
-        sys.path[0] = sys.path[0] + "/../"
-        import debugpy  # noqa
+
+        # if the user has specified a path to the debugpy module, replace sys.path[0] with
+        # the specified path. Otherwise, replace sys.path[0] with the parent directory of debugpy/
+        debugpy_path = os.environ.get("DEBUGPY_PATH")
+        if (debugpy_path is not None):    
+            sys.path[0] = debugpy_path
+        else:
+            # Do not use dirname() to walk up - this can be a relative path, e.g. ".".
+            sys.path[0] = sys.path[0] + "/../"
+            import debugpy  # noqa
 
         del sys.path[0]