## mxQueue - A Queue Implementation for Python

Interface : Examples : C API : Structure : Support : Download : Copyright & License : History : Home | Version 2.0.3

### Introduction

XXX This documentation is unfinished... it is basically a search&replace copy of the mxStack documentation since the mxQueue APIs are very similar to mxStack. Usage should be straight forward though. The next release will have proper documentation and also be advertised on the download page :-).

Though queues can be emulated with Python lists, this type provides a simple interface to the data structure, both in Python and in C. Because of the function call overhead calling the methods from Python it is only a tad faster than a corresponding list emulation. Called from within an C extension shows a more significant performance increase. The included `queuebench.py` gives an impression of how the different methods relate w/r to speed:

    projects/Queue> python1.5 -O queuebench.py 1000 100 100
    list:  1.11
    tuples: 0.6
    Queue (with push + pop): 0.72
    Queue (with push + pop_many): 0.5
    Queue (with << + >>): 0.85
    Queue (with push_many + pop_many): 0.47
    UserQueue: 1.79

Note that the tuple version has a few disadvantages when used for big queues: for one it uses lots of memory (20 bytes per entry slot; Queue uses 20 bytes + 4 bytes per entry slot) and deallocation can become a problem -- this is done using recursion with one level per queue element. For small queues it still is unbeatable, though (it has no function call overhead). BTW, the UserQueue implementation uses the same technique: the figures shown mainly result from Python method call overhead.

Because queues are normally used only temporarily, the Queue implementation only grows the memory buffer used for holding the entry slots. It never shrinks it. This has an advantage of reducing malloc overhead when doing e.g. depth first search, but also the disadvantage of using more memory in degenerate cases. To compensate for this, simply call the .resize() method every now and then. It forces the used buffer to be resized.

### Interface

Queue Constructors

There are two ways to construct a `Queue` from scratch:

`Queue([initial_size])`
  Returns a new empty Queue instance allocating at least the given number of slots for queue elements. If the parameter is not given a reasonable default is chosen.

`QueueFromSequence(seq)`
  Constructs a Queue instance from the given sequence. The instance is filled with all the elements found in the sequence by pushing the items from index 0 to len(seq)-1 in that order, i.e. popping all elements from the Queue results in a reversed sequence.

Instance Methods

A `Queue` instance has the following methods:

`push(x)`
  Pushes the object x onto the queue.

`push_many(sequences)`
  Pushes the objects in `sequence` from left to right onto the queue. If errors occur during this process, the already pushed elements are discarded from the queue and it returns to its original state.

`pop()`
  Pops the top element off of the queue.

`pop_many(n)`
  Pops the top `n` elements and returns them in form of a tuple. If less than `n` elements are on the queue, the tuple will contain all queue entries and the queue will then be empty again. The order is top to bottom, i.e. `s.pop_many(2) == (s.pop(),s.pop())`

`as_tuple()`
  Returns the queue's content as tuple, without modifying it.

`as_list()`
  Returns the queue's content as list, without modifying it.

`clear()`
  Clears the queue.

`resize([size=len(queue)])`
  Resize the queue buffer to hold at least `size` entries.

  You can call this method without argument to force the queue to shrink its memory buffer to the minimal limit needed to hold the contained elements.

Note that no method for testing emtpyness is provided. Use len() for that or simply test for trueness, e.g. `while s: print s.pop()` will loop as long as there are elements left on the Queue s. This is much faster than going through the method calling process -- even when the method being called is written in C.

### Examples of Use

Well, there's not much to show:

    from mx.Queue import *
    s = Queue()
    for i in range(1000):
        s.push(i)
    while s:
        print s.pop()
    # which could also be done as:
    s = QueueFromSequence(range(1000))
    while s:
        print s.pop()
    # or a little different
    s = QueueFromSequence(range(1000))
    print s.as_tuple()
    print s.as_list()

### Supported Data Types in the C-API

Please have look at the file `mxQueue.h` for details. Basically all of the above Python interfaces are also available in the C API.

To access the module, do the following (note the similarities with Python's way of accessing functions from a module):

    #include "mxQueue.h"

    ...
        PyObject *v;

        /* Import the mxQueue module */
        if (mxQueue_ImportModuleAndAPI())
    	goto onError;

        /* Access functions from the exported C API through mxQueue */
        v = mxQueue.Queue(0);
        if (!v)
    	goto onError;

        /* Type checking */
        if (mxQueue_Check(v))
            printf("Works.\n");

        Py_DECREF(v);
    ...

### Package Structure

    [Queue]
    	mxQueue

Entries enclosed in brackets are packages (i.e. they are directories that include a `__init__.py` file). Ones without brackets are just simple subdirectories that are not accessible via `import`. These are used for compiling the C extension modules which will get installed in the same place where all your other site specific extensions live (e.g. `/usr/local/lib/python-x.xx/site-packages`).

The package Queue imports all symbols from the extension mxQueue, so `import Queue; s = Queue.Queue()` gives you a Queue instance in `s`.

### Support

eGenix.com is providing commercial support for this package. If you are interested in receiving information about this service please see the eGenix.com Support Conditions.

### Copyright & License

© 1999-2000, Copyright by Marc-André Lemburg; All Rights Reserved. mailto: mal@lemburg.com

© 2000-2001, Copyright by eGenix.com Software GmbH, Langenfeld, Germany; All Rights Reserved. mailto: info@egenix.com

This software is covered by the eGenix.com Public License Agreement. The text of the license is also included as file "LICENSE" in the package's main directory.

By downloading, copying, installing or otherwise using the software, you agree to be bound by the terms and conditions of the eGenix.com Public License Agreement.

### History & Future

There were no significant changes between 2.0.2 and 2.0.3.

Changes from version 2.0.0 to 2.0.2:

- Fixed a bug in the coercion code which surfaced due to the rich comparison changes in Python 2.1. Python 2.1 will now compare Queue objects to other objects without raising a TypeError.

Version 2.0.0 was the initial public version.

© 1999-2000, Copyright by Marc-André Lemburg; All Rights Reserved. mailto: mal@lemburg.com

© 2000-2001, Copyright by eGenix.com Software GmbH; All Rights Reserved. mailto: info@egenix.com