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Quasiquotation in python

Project description

docs

Blocks of non-python code sprinkled in for extra seasoning.

What is a quasiquote

An quasiquote is a new syntactical element that allows us to embed non python code into our existing python code. The basic structure is as follows:

# coding: quasiquotes

[$name|some code goes here|]

This desuagars to:

name.quote_expr("some code goes here", frame, col_offset)

where frame is the executing stack frame and col_offset is the column offset of the quasiquoter.

This allows us to use slightly nicer syntax for our code. The # coding: quasiquotes is needed to enable this extension. The syntax is chosen to match haskell’s quasiquote syntax from GHC 6.12. We need to use the older syntax (with the $) because python’s grammar would be ambiguous without it at the quote open step. To simplify the tokenizer, we chose to use slighly more verbose syntax.

We may also use statement syntax for quasiquotes in a modified with block:

# coding: quasiquotes

with $name:
    some code goes here

This desuagars to:

name.quote_stmt("    some code goes here", frame, col_offset)

The c quasiquoter

The builtin c quasiquoter allows us to inline C code into our python. For example:

>>> from quasiquotes.c import c
>>> def f(a):
...     with $c:
...         printf("%ld\n", PyLong_AsLong(a));
...         a = Py_None;
...         Py_INCREF(a);
...     print(a)
...
>>> f(0)
0
None
>>> f(1)
1
None

Here we can see that the quasiquoter can read from and write to the local scope.

We can also quote C expressions with the quote expression syntax.

>>> def cell_new(n):
...     return [$c|PyCell_New(n);]
...
>>> cell_new(1)
<cell at 0x7f8dde6cd5e8: int object at 0x7f8ddf956780>

Here we can see that the c quasiquoter is really convenient as a python interface into the C API.

Warning

CPython uses a reference counting system to manage the lifetimes of objects. Code like:

return [$|Py_None|]

can cause a potential segfault when None because it will have 1 less reference than expected. Instead, be sure to remember to incref your expressions with:

return [$|Py_INCREF(Py_None); Py_None|]

You must also incref when reassigning names from the enclosing python scope. For more information, see the CPython docs.

IPython Integration

We can use the c quasiquoter in the IPython repl or notebook as a cell or line magic. When used as a line magic, it is quoted as an expression. When used as a cell magic, it is quoted as a statement.

In [1]: import quasiquotes.c

In [2]: a = 5

In [3]: %c PyObject *b = PyLong_FromLong(3); PyObject *ret = PyNumber_Add(a, b); Py_DECRE   F(b); ret;
Out[3]: 8

In [4]: %%c
   ...: printf("%ld + %ld = %ld\n", 3, PyLong_AsLong(a), PyLong_AsLong(_3));
   ...: puts("reassigning 'a'");
   ...: a = Py_None;
   ...: Py_INCREF(a);
   ...:
3 + 5 = 8
reassigning 'a'

In [5]: a is None
Out[5]: True

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