# Using Python

KLayout does not come with one integrated interpreter. Instead Python and Ruby can both be used together. So it is possible to write one script in Ruby and another one in Python. Just pick your favorite language. Scripts written in different languages share the same KLayout data structures. Naturally they cannot directly share variables or language-specific data. But you can, for example, implement PCell's in Python and Ruby and use those different PCells in the same layout at the same time. Depending on the type of PCell, KLayout will either execute Python or Ruby code.

Python macros are loaded into KLayout using either ".py" files or ".lym" files with the interpreter set to "Python". To create Python macros, a new tab is available in the Macro Development IDE. When creating macros in the "Python" tab, they will use the Python interpreter. Macros created in the "Ruby" tab will use the Ruby interpreter. Files loaded by "import" need to be in plain text format and use the ".py" suffix. The macro folder is called "pymacros" for a clean separation between the two macro worlds. Technically, both Ruby and Python macros are .lym files with a different interpreter specified in these files.

The Python macro folder is in the "sys.path" search path so it is possible to install modules there. To install libraries globally use "inst_path/lib/python/Lib" and "inst_path/lib/python/DLLs" on Windows. inst_path is the installation path (where klayout.exe is located). On Linux, the installation will share the Python interpreter with the system and modules installed there will be available for KLayout too.

"$PYTHONHOME" is not supported to prevent interference with other Python consumers. Instead, KLayout will read the Python path from "$KLAYOUT_PYTHONPATH" (for Python >= 3.x).

## Writing Macros in Python

A good way is to start with the samples provided when creating new macros on the Python tab. The samples are available at the end of the template list. There is a sample for a PCell implementation, a sample for a Qt dialog, a sample for using Qt's .ui files in Python macros and one sample turning KLayout into a HTTP server using a Python macro.

Apart from a few specialities and the different language of course, Python macros do not look much different from Ruby macros. Ruby's "RBA" namespace is "pya" for Python (lowercase to conform with PEP-8). The class and methods names are the same with very few exceptions and the documentation can be used for Python too. Where necessary, a special remark is made regarding the Python implementation.

Here is a basic Python Macro. It creates a layout with a single cell and single layer and puts one rectangle on that layer:

    # Python version:

    import pya

    layout = pya.Layout()
    top = layout.create_cell("TOP")
    l1 = layout.layer(1, 0)
    top.shapes(l1).insert(pya.Box(0, 0, 1000, 2000))

    layout.write("t.gds")

Here is the Ruby variant to demonstrate the similarity:

    # Ruby version:

    layout = RBA::Layout::new()
    top = layout.create_cell("TOP")
    l1 = layout.layer(1, 0)
    top.shapes(l1).insert(RBA::Box::new(0, 0, 1000, 2000))

    layout.write("t.gds")

Of course, not everything can be translated that easily between Ruby and Python. The details are given below. Usually however, it's straightforward to translate Ruby into Python.

There is no clear advantage of one language over the other. The Python community is somewhat stronger, but performance-wise, Ruby is better. In KLayout, the debugger support for Python is slighly better, since the guts of the interpreter are better documented for Python.

Apart from that, Python and Ruby coexist remarkably well and it is amazing, how easy it it to extend the interfaces from Ruby to Python: not counting the different in the memory management model (mark and sweep garbage collector in Ruby, reference counting in Python), the concepts are very similar.

Please read the Python specific notes below before you start. Some things need to be considered when going from Ruby to Python.

## Python PCells

Please have a look at the PCell sample available in the templates. Pick the PCell sample after you have created a new Python macro.

PCell implementation in Python is very similar to Ruby.

Python macros are ".lym" files that are placed into the "pymacro" subfolder in the KLayout path. Python libraries can be put into the "python" subfolder. This subfolder is included into the "sys.path" variable, so macros can load libraries simply by using "import".

## Python Implementation Notes

- KLayout module: KLayout's module is "pya" (lowercase conforming to PEP 8).
- Reserved names: Some methods with reserved names are not available (i.e. "exec", "in"). Some of these methods have been renamed and their original name is still available in Ruby, but use is deprecated. Where this was not possible, they are available with an appended underscore. For example: "QDialog.exec" is available as "QDialog.exec_". That is the same scheme PyQt uses.
- Assignment methods (attribute setters): Assignment methods (i.e. "Box#left=" are available as attributes. If there is a read accessor method too, the attribute can be read and written. For example:

      box = pya.Box()
      box.left = 10
      box.right = box.right + 100

  If the translation is ambiguous (i.e. because there is more than one getter or setter, the setter will be translated to a method "set_x(value)" where "x" is the attribute name.

- Predicate getters: Question-mark names for predicates are translated to non-question-marker names:

      # Ruby:
        edges.is_empty?

      # Python:
        edges.is_empty()

- Constants: Constants (upper-case static variable) are made available as static attributes.
- Arrays: Arrays will be represented as lists, but on assignment, they accept tuples as well.
- Boolean values: Boolean values are True and False.
- No protected methods: Protected methods are not supported - methods are public always.
- "nil" value: The Python equivalent to Ruby's "nil" is "None".
- Iterators: Iterator binding:

      edges = pya.Edges()
      ...
      for edge in edges.each():
        ...

  If there is an iterator named "each", it will become the default iterator:

      for edge in edges:
        ...

- Standard protocols:

  "x.to_s()" is available as "str(x)" too.

  "x.size()" is available as "len(x)" too.

  If there is a "[]" operator and a "size" method, the object implements the sequence protocol too.

- Operators:

  Operators are made available through Python operators. For example

  - "+" will be available as "+" ("__add__")
  - "&" will be available as "and" ("__and__")
  - "|" will be available as "or" ("__or__")
  - "==" will be available as "==" ("__eq__")
- Deep copies: Deep copies of pya objects can be made with dup()

      box = pya.Box(10, 20, 110, 220)
      copy_box = box.dup()

- Events (signals):

  Events can be bound to lambdas or functions:

      action.on_triggered( lambda: action.text += "X" )

  or to function:

      def f():
        print "triggered"

      action.on_triggered(f)

  Events have to match precisely - exactly the number of arguments have to be declared.

- sys.settrace: Using "sys.settrace" will disable the debugger support permanently.
- Instance attributes can't reimplement virtual methods: This is a limitation driven by the need to avoid cyclic references. Instance-bound methods require a reference to the instance and that will create a cycle with the reimplementation callable object which is held by the class itself.

- Tips when developing own modules:

  - The "python" subfolders of the KLayout path are added to sys.path, so modules can be put as plain .py files and imported with "import module".
  - Or: modules can be put into folders inside "python" using an "__init__.py" file to indicate the folder is a module.
  - Use "reload(module)" on the console to refresh the module cache if changes have been applied.