Docs: Hy <-> Python interop fix #1061 (#1218)

* Docs: Hy <-> Python interop fix #1061

I separated the existing text in two sections, with some additional
explanations and a link to Hy's `import`.

* added interop page

* simplified interop section in tutorial

* remove the TODO from interop section in tutorial

Because yay

* Corrections from @Kodiologist

I kept the part about mangling, but added a warning about its incompleteness. I
think it can be useful for somebody who just wants to use a Python module in his
code. Maybe it can be removed when the actual documentation for mangling is
written.

* Added myself to AUTHORS.

I'll do my best to be worthy of it. Thanks for this awesome project!

AUTHORS

@@ -74,3 +74,4 @@
 * Karan Sharma <[email protected]>
 * Sergey Sobko <[email protected]>
 * Philip Xu <[email protected]>
+* Charles de Lacombe <[email protected]>

docs/language/api.rst

@@ -1049,6 +1049,7 @@ reverses the comparison.
     => (lif-not False "true" "false")
     "false"
 
+.. _import:
 
 import
 ------

docs/language/index.rst

@@ -8,6 +8,7 @@ Contents:
    :maxdepth: 3
 
    cli
+   interop
    api
    core
    readermacros

docs/language/interop.rst

@@ -0,0 +1,135 @@
+=====================
+Hy <-> Python interop
+=====================
+
+   “Keep in mind we’re not Clojure. We’re not Common Lisp. We’re Homoiconic
+   Python, with extra bits that make sense.” — Hy Style Guide
+
+Despite being a Lisp, Hy aims to be fully compatible with Python. That means
+every Python module or package can be imported in Hy code, and vice versa.
+
+Using Python from Hy
+====================
+
+Using Python from Hy is nice and easy, you just have to :ref:`import` it.
+
+If you have the following in ``greetings.py`` in Python::
+
+    def greet(name):
+        print("hello," name)
+
+You can use it in Hy:
+
+.. code-block:: clj
+
+    (import greetings)
+    (.greet greetings "foo") ; prints "hello, foo"
+
+You can also import ``.pyc`` bytecode files, of course.
+
+A quick note about mangling
+--------
+
+In Python, snake_case is used by convention. Lisp dialects tend to use dashes
+instead of underscores, so Hy does some magic to give you more pleasant names.
+
+In the same way, ``UPPERCASE_NAMES`` from Python can be used ``*with-earmuffs*``
+instead.
+
+You can use either the original names or the new ones.
+
+Imagine ``example.py``::
+
+    def function_with_a_long_name():
+        print(42)
+
+    FOO = "bar"
+
+Then, in Hy:
+
+.. code-block:: clj
+
+    (import example)
+    (.function-with-a-long-name example) ; prints "42"
+    (.function_with_a_long_name example) ; also prints "42"
+
+    (print (. example *foo*)) ; prints "bar"
+    (print (. example FOO))   ; also prints "bar"
+
+.. warning::
+   Mangling isn’t that simple; there is more to discuss about it, yet it doesn’t
+   belong in this section.
+.. TODO: link to mangling section, when it is done
+
+
+Using Hy from Python
+====================
+
+Suppose you have written some useful utilities in Hy, and you want to use them in
+regular Python, or to share them with others as a package. Or suppose you work
+with somebody else, who doesn't like Hy (!), and only uses Python.
+
+In any case, you need to know how to use Hy from Python. Fear not, for it is
+easy.
+
+If you save the following in ``greetings.hy``:
+
+.. code-block:: clj
+
+    (setv *this-will-be-in-caps-and-underscores* "See?")
+    (defn greet [name] (Print "hello from hy," name))
+
+Then you can use it directly from Python, by importing Hy before importing
+the module. In Python::
+
+    import hy
+    import greetings
+
+    greetings.greet("Foo") # prints "Hello from hy, Foo"
+    print(THIS_WILL_BE_IN_CAPS_AND_UNDERSCORES) # prints "See?"
+
+If you create a package with Hy code, and you do the ``import hy`` in
+``__init__.py``, you can then directly include the package. Of course, Hy still
+has to be installed.
+
+Compiled files
+--------------
+
+You can also compile a module with ``hyc``, which gives you a ``.pyc`` file. You
+can import that file. Hy does not *really* need to be installed ; however, if in
+your code, you use any symbol from :doc:`core`, a corresponding ``import``
+statement will be generated, and Hy will have to be installed.
+
+Even if you do not use a Hy builtin, but just another function or variable with
+the name of a Hy builtin, the ``import`` will be generated. For example, the previous code
+causes the import of ``name`` from ``hy.core.language``.
+
+**Bottom line: in most cases, Hy has to be installed.**
+
+Launching a Hy REPL from Python
+-------------------------------
+
+You can use the function ``run_repl()`` to launch the Hy REPL from Python::
+
+    >>> import hy.cmdline
+    >>> hy.cmdline.run_repl()
+    hy 0.12.1 using CPython(default) 3.6.0 on Linux
+    => (defn foo [] (print "bar"))
+    => (test)
+    bar
+
+If you want to print the Python code Hy generates for you, use the ``spy``
+argument::
+
+    >>> import hy.cmdline
+    >>> hy.cmdline.run_repl(spy=True)
+    hy 0.12.1 using CPython(default) 3.6.0 on Linux
+    => (defn test [] (print "bar"))
+    def test():
+        return print('bar')
+    => (test)
+    test()
+    bar
+
+
+

docs/tutorial.rst

@@ -8,7 +8,6 @@ Tutorial
 ..  - How do I do array ranges?  e.g. x[5:] or y[2:10]
 ..  - Blow your mind with macros!
 ..  - Where's my banana???
-..  - Mention that you can import .hy files in .py files and vice versa!
 
 Welcome to the Hy tutorial!
 
@@ -585,56 +584,43 @@ compile-time. ``require`` uses the same syntax as ``import``.
 Hy <-> Python interop
 =====================
 
-By importing Hy, you can use Hy directly from Python!
+Using Hy from Python
+--------------------
+
+You can use Hy modules in Python!
 
 If you save the following in ``greetings.hy``:
 
 .. code-block:: clj
 
     (defn greet [name] (print "hello from hy," name))
 
-Then you can use it directly from python, by importing hy before importing
+Then you can use it directly from Python, by importing Hy before importing
 the module. In Python::
 
     import hy
     import greetings
 
     greetings.greet("Foo")
 
-You can also declare a function in python (or even a class!) and use it in Hy!
+Using Python from Hy
+--------------------
+
+You can also use any Python module in Hy!
 
 If you save the following in ``greetings.py`` in Python::
 
     def greet(name):
         print("hello, %s" % (name))
 
-You can use it in Hy:
+You can use it in Hy (see :ref:`import`):
 
 .. code-block:: clj
 
     (import greetings)
     (.greet greetings "foo")
 
-To use keyword arguments, you can use in ``greetings.py``::
-
-    def greet(name, title="Sir"):
-        print("Greetings, %s %s" % (title,name))
-
-.. code-block:: clj
-
-    (import greetings)
-    (.greet greetings "Foo")
-    (.greet greetings "Foo" "Darth")
-    (apply (. greetings greet) ["Foo"] {:title "Lord"})
-
-Which would output::
-
-  Greetings, Sir Foo
-
-  Greetings, Darth Foo
-
-  Greetings, Lord Foo
-
+More information on :doc:`../language/interop`.
 
 
 Protips!