first draft of using parameters

api/docs/v2/new_protocol_api.rst

@@ -14,7 +14,7 @@ Protocols
 
 .. autoclass:: opentrons.protocol_api.ProtocolContext
    :members:
-   :exclude-members: location_cache, cleanup, clear_commands, params
+   :exclude-members: location_cache, cleanup, clear_commands
 
 Instruments
 ===========

api/docs/v2/parameters/using_values.rst

@@ -6,13 +6,67 @@
 Using Parameters
 ****************
 
-The ``params`` object
+Once you've :ref:`defined parameters <defining-rtp>`, their values are accessible anywhere within the ``run()`` function of your protocol.
+
+The ``params`` Object
 =====================
 
-Other topics TK
+Protocols with parameters have a :py:obj:`.ProtocolContext.params` object, which contains the values of all parameters as set during run setup. Each attribute of ``params`` corresponds to the ``variable_name`` of a parameter.
+
+For example, consider a protocol that defines the following three parameters:
+
+- ``add_bool`` with ``variable_name="dry_run"``
+- ``add_int`` with ``variable_name="sample_count"``
+- ``add_float`` with ``variable_name="volume"``
+
+Then ``params`` will gain three attributes: ``params.dry_run``, ``params.sample_count``, and ``params.volume``. You can use these attributes anywhere you want to access their values, including directly as arguments of methods.
+
+.. code-block::
+
+    if protocol.params.dry_run == False:
+        pipette.mix(repetitions=10, volume=protocol.params.volume)
+
+You can also save parameter values to variables with names of your choosing.
+
+Parameter Types
 ===============
 
-To fill in later:
+Each attribute of ``params`` has the type corresponding to its parameter definition. Keep in mind the parameter's type when using its value in different contexts.
+
+Say you wanted to add a comment to the run log, stating how many samples the protocol will process. Since ``sample_count`` is an ``int``, you'll need to cast it to a ``str`` or the API will raise an error.
+
+.. code-block::
+
+    protocol.comment(
+        "Processing " + str(protocol.params.sample_count) + " samples."
+    )
+
+Also be careful with ``int`` types when performing calculations: dividing an ``int`` by an ``int`` with the ``/`` operator always produces a ``float``, even if there is no remainder. The :ref:`sample count use case <use-case-sample-count>` converts a sample count to a column count by dividing by 8 — but it uses the ``//`` integer division operator, so the result can be used for creating ranges, slicing lists, and as ``int`` argument values without having to cast it in those contexts.
+
+Limitations
+===========
+
+Since ``params`` is only available within the ``run()`` function, there are certain aspects of a protocol that parameter values can't affect. These include, but are not limited to the following:
+
+
+.. list-table::
+    :header-rows: 1
 
-- Type checking
-- TKTKTK
+    * - Information
+      - Location
+    * - ``import`` statements
+      - At the beginning of the protocol.
+    * - Robot type (Flex or OT-2)
+      - In the ``requirements`` dictionary.
+    * - API version
+      - In the ``requirements`` or ``metadata`` dictionary.
+    * - Protocol name
+      - In the ``metadata`` dictionary.
+    * - Protocol description
+      - In the ``metadata`` dictionary.
+    * - Protocol author
+      - In the ``metadata`` dictionary.
+    * - Other runtime parameters
+      - In the ``add_parameters()`` function.
+    * - Non-nested function definitions
+      - Anywhere outside of ``run()``.