Update docs

docs/interop/python.md

@@ -39,6 +39,16 @@ print(multiply(3, 4))  # 12
 (Be sure the `PYTHONPATH` environment variable includes the path of
 *mymodule.py*!)
 
+`from python import` does not need to specify explicit types, in which case
+Codon will operate directly on the Python objects, and convert Codon types
+to Python types as necessary:
+
+``` python
+from python import numpy as np  # Codon will call NumPy through CPython's API
+x = np.array([1, 2, 3, 4]) * 10
+print(x)  # [10 20 30 40]
+```
+
 # `@python`
 
 Codon programs can contain functions that will be executed by Python via
@@ -62,3 +72,24 @@ def myrange(n: int) -> List[int]:
 
 print(myrange(5))  # [0, 1, 2, 3, 4]
 ```
+
+# Data conversions
+
+Codon uses two new magic methods to transfer data to and from Python:
+
+- `__to_py__`: Produces a Python object (`PyObject*` in C) given a Codon object.
+- `__from_py__`: Produces a Codon object given a Python object.
+
+``` python
+import python  # needed to initialize the Python runtime
+
+o = (42).__to_py__()  # type of 'o' is 'cobj', equivalent to a pointer in C
+print(o)  # 0x100e00610
+
+n = int.__from_py__(o)
+print(n)  # 42
+```
+
+Codon stores the results of `__to_py__` calls by wrapping them in an instance of
+a new class called `pyobj`, which correctly handles the underlying Python object's
+reference count. All operations on `pyobj`s then go through CPython's API.