Write query_modules/example.c as example.py

Summary:
This is an overview on how the example Query Module will look in
Python.

Reviewers: mferencevic, ipaljak, buda, tlastre, dsantl

Reviewed By: ipaljak, tlastre, dsantl

Subscribers: pullbot

Differential Revision: https://phabricator.memgraph.io/D2642

query_modules/example.py

@@ -0,0 +1,37 @@
+# Build Memgraph with Python 3.5+ support and run Memgraph so that it loads
+# this file as a Query Module. The procedure implemented in this module is just
+# a rewrite of the `example.c` procedure.
+import mgp
+
+import copy
+
+# mgp.read_proc will use inspect.signature to get the arguments and types which
+# we map to our C API. Alternative registration examples are written below
+# this definition. Also, we can support other builtin types that make sense, as
+# well as the `typing` module, but `typing.Any` maps to `mgp.Nullable(mgp.Any)`
+# instead of `mgp.Any` which prevents `None`. Additionally, all `mgp.read_proc`
+# decorator registration relies on the assumption that they will have a
+# globally visible `mgp_module` on which procedures are registered. For this to
+# work, importing Python modules as Query Modules will need to go through a
+# single thread or should use a thread local variable. Currently, we load
+# everything on startup in a single thread, so there are no issues.
+@mgp.read_proc
+def procedure(context: mgp.ProcCtx,
+              required_arg: mgp.Nullable[mgp.Any],
+              optional_arg: mgp.Nullable[mgp.Any] = None
+              ) -> mgp.Record(result=str, args=list):
+    '''
+    This example procedure returns 2 fields: `args` and `result`.
+      * `args` is a copy of arguments passed to the procedure.
+      * `result` is the result of this procedure, a "Hello World!" string.
+    Any errors can be reported by raising an Exception.
+
+    The procedure can be invoked in openCypher using the following calls:
+      CALL example.procedure(1, 2) YIELD args, result;
+      CALL example.procedure(1) YIELD args, result;
+    Naturally, you may pass in different arguments or yield less fields.
+    '''
+    # Copying to make it equivalent to the C example.
+    args_copy = [copy.deepcopy(required_arg), copy.deepcopy(optional_arg)]
+    # Multiple rows can be produced by returning an iterable of mgp.Record
+    return mgp.Record(args=args_copy, result='Hello World!')