Introduce namedtuple types for position and size

[freakboy3742]

What is the problem or limitation you are having?

Toga makes extensive use of tuples to pass around size and position values (e.g., the size and position of a window)

Tuples work for this purpose, and are extremely convenient from the perspective of the user providing input; but can make usage obscure because they rely on understanding that, for example mysize[0] is width, and mysize[1] is height.

Describe the solution you'd like

Toga should define namedtuples for Size and Position data types. This would allow the usage of mysize.width and mysize.height, myposition.x and myposition.y at runtime.

These type declarations should be in a toga.types package, but exposed through the toga namespace, so toga.Size() and toga.Position() would be legal references.

Any setter or method that accepts a tuple should continue to do so in addition to the new type; however, internally, Toga should cast the "raw" tuple to the named tuple as early as is convenient. Any value returned by a Toga API should be in the namedtuple form.

A SizeT type alias for "Size in nametuple or raw tuple form" should be defined (and similar for position), so that there's a convenient way for inputs to declare that both forms are valid. SizeT would only be used for input; output should always have the type annotation toga.Size.

Any internal code referencing size[0] could then be updated to reference size.width etc; however, this should be considered optional, as the existing size[0] usage should continue to work.

Describe alternatives you've considered

• Do nothing, and continue to use raw tuples
• Use a full class, rather than a namedtuple. This essentially gains no benefit over namedtuple, other than needing to define all the extra methods to make the new class behave like a tuple.

Additional context

Care should be taken to ensure that backends return data in the new types, rather than tuples. Adding type checks to the testbed backend may be advisable to ensure that the returned types are always honoured.

[Dundy703]

Could I give this a shot?

[freakboy3742]

@Dundy703 Go right ahead! We've got a contribution guide to help step you through the process of setting up a development environment and generating your first PR. If you've got any questions, let us know.

[freakboy3742]

By coincidence, I've needed to introduce a LatLng namedtuple type as part of #2379; that PR hasn't merged yet, but anyone implementing a fix for this issue may find that PR helpful as a guide - most of the infrstructure introduced there will be identical to what is needed by this issue.

• A LatLng namedtuple
• Exposing types into the toga namespace
• Using a namedtuple type
• Documentation for a namedtuple
• Documentation configuration to support namedtuples

[rashenck]

Going to picking this up at the Pycon US sprint!