Docs: Introduce pure Python mode in "Using C libraries" tutorial (GH-4294)

docs/examples/tutorial/clibraries/cqueue.pxd

-# cqueue.pxd
-
 cdef extern from "c-algorithms/src/queue.h":
     ctypedef struct Queue:
         pass

docs/examples/tutorial/clibraries/queue.py

+from cython.cimports import cqueue
+
+@cython.cclass
+class Queue:
+    _c_queue = cython.declare(cython.pointer(cqueue.Queue))
+
+    def __cinit__(self):
+        self._c_queue = cqueue.queue_new()

docs/examples/tutorial/clibraries/queue.pyx

-# queue.pyx
-
 cimport cqueue
 
+
 cdef class Queue:
     cdef cqueue.Queue* _c_queue
 

docs/examples/tutorial/clibraries/queue2.py

+from cython.cimports import cqueue
+
+@cython.cclass
+class Queue:
+    _c_queue = cython.declare(cython.pointer(cqueue.Queue))
+
+    def __cinit__(self):
+        self._c_queue = cqueue.queue_new()
+        if self._c_queue is cython.NULL:
+            raise MemoryError()

docs/examples/tutorial/clibraries/queue2.pyx

-# queue.pyx
-
 cimport cqueue
 
+
 cdef class Queue:
     cdef cqueue.Queue* _c_queue
 

docs/examples/tutorial/clibraries/queue3.py

+from cython.cimports import cqueue
+from cython import cast
+
+@cython.cclass
+class Queue:
+    """A queue class for C integer values.
+
+    >>> q = Queue()
+    >>> q.append(5)
+    >>> q.peek()
+    5
+    >>> q.pop()
+    5
+    """
+    _c_queue = cython.declare(cython.pointer(cqueue.Queue))
+    def __cinit__(self):
+        self._c_queue = cqueue.queue_new()
+        if self._c_queue is cython.NULL:
+            raise MemoryError()
+
+    def __dealloc__(self):
+        if self._c_queue is not cython.NULL:
+            cqueue.queue_free(self._c_queue)
+
+    @cython.ccall
+    def append(self, value: cython.int):
+        if not cqueue.queue_push_tail(self._c_queue,
+                cast(cython.p_void, cast(cython.Py_ssize_t, value))):
+            raise MemoryError()
+
+    # The `cpdef` feature is obviously not available for the original "extend()"
+    # method, as the method signature is incompatible with Python argument
+    # types (Python does not have pointers).  However, we can rename
+    # the C-ish "extend()" method to e.g. "extend_ints()", and write
+    # a new "extend()" method that provides a suitable Python interface by
+    # accepting an arbitrary Python iterable.
+    @cython.ccall
+    def extend(self, values):
+        for value in values:
+            self.append(value)
+
+    @cython.cfunc
+    def extend_ints(self, values: cython.p_int, count: cython.size_t):
+        value: cython.int
+        for value in values[:count]:  # Slicing pointer to limit the iteration boundaries.
+            self.append(value)
+
+    @cython.ccall
+    @cython.exceptval(-1, check=True)
+    def peek(self) -> cython.int:
+        value: cython.int = cast(cython.Py_ssize_t, cqueue.queue_peek_head(self._c_queue))
+
+        if value == 0:
+            # this may mean that the queue is empty,
+            # or that it happens to contain a 0 value
+            if cqueue.queue_is_empty(self._c_queue):
+                raise IndexError("Queue is empty")
+        return value
+
+    @cython.ccall
+    @cython.exceptval(-1, check=True)
+    def pop(self) -> cython.int:
+        if cqueue.queue_is_empty(self._c_queue):
+            raise IndexError("Queue is empty")
+        return cast(cython.Py_ssize_t, cqueue.queue_pop_head(self._c_queue))
+
+    def __bool__(self):
+        return not cqueue.queue_is_empty(self._c_queue)

docs/examples/tutorial/clibraries/queue3.pyx

-# queue.pyx
-
 cimport cqueue
 
+
+
 cdef class Queue:
     """A queue class for C integer values.
 
@@ -22,6 +22,7 @@ cdef class Queue:
         if self._c_queue is not NULL:
             cqueue.queue_free(self._c_queue)
 
+
     cpdef append(self, int value):
         if not cqueue.queue_push_tail(self._c_queue,
                                       <void*> <Py_ssize_t> value):
@@ -33,15 +34,19 @@ cdef class Queue:
     # the C-ish "extend()" method to e.g. "extend_ints()", and write
     # a new "extend()" method that provides a suitable Python interface by
     # accepting an arbitrary Python iterable.
+
     cpdef extend(self, values):
         for value in values:
             self.append(value)
 
+
     cdef extend_ints(self, int* values, size_t count):
         cdef int value
         for value in values[:count]:  # Slicing pointer to limit the iteration boundaries.
             self.append(value)
 
+
+
     cpdef int peek(self) except? -1:
         cdef int value = <Py_ssize_t> cqueue.queue_peek_head(self._c_queue)
 
@@ -52,6 +57,8 @@ cdef class Queue:
                 raise IndexError("Queue is empty")
         return value
 
+
+
     cpdef int pop(self) except? -1:
         if cqueue.queue_is_empty(self._c_queue):
             raise IndexError("Queue is empty")