使用python编写量子线路打印的简单项目，并使用Sphinx自动化生成API文档

技术背景

该文章一方面从量子线路的打印着手，介绍了一个简单的python量子线路工程。同时基于这个简单的小工程，我们顺带的介绍了python的API文档自动化生成工具Sphinx的基本使用方法。

量子线路背景知识

在前面几篇博客中，有介绍过使用开源量子计算编程框架ProjectQ进行量子线路的绘制，会给我们输出一个tex格式的线路图，在文章中可以直接使用。关于量子线路与量子逻辑门操作，在这篇博客中有比较初步的介绍。而本文章中所创建的工程，是直接在cmd窗口里面打印输出字符串形式的量子线路，同样的，在量子计算资源估计和量子线路工程中，可以产生一定的作用。

自动化文档生成的方案

对于一个比较优雅的python开源项目来说，一份简介的文档是必不可少的。一般一个python项目的文档有两部分组成：一部分是用markdown撰写的使用说明文档，其宗旨在于概述的介绍整个项目的重点内容，以及可能包含少部分的使用示例。这部分的文档可以先用markdown写，然后通过一些开源工具，如mkdocs等转换成read_the_docs格式的文档。read_the_docs格式的文档大概如下图所示，也可以直接参考其官方文档。而文档的第二个部分则是具体到每个函数、每个类的接口文档。在开发阶段，我们先按照格式要求写好注释文档，然后通过开源工具Sphinx就可以自动化的生成API接口文档。

安装sphinx

这里我们直接使用python的包管理工具pip来安装Sphinx以及一个read_the_docs格式的python库。如果不需要使用read_the_docs格式也可以不安装后者，只是后者在python的开源项目中还是最常用的一种文档格式，并且可以配合read_the_docs网站进行API文档的托管，因此推荐使用。

[dechin@dechin-manjaro circuit]$ python3 -m pip install sphinx
Requirement already satisfied: sphinx in /home/dechin/anaconda3/lib/python3.8/site-packages (3.4.3)
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[dechin@dechin-manjaro circuit]$ python3 -m pip install sphinx-rtd-theme
Collecting sphinx-rtd-theme
  Downloading sphinx_rtd_theme-0.5.1-py2.py3-none-any.whl (2.8 MB)
     |████████████████████████████████| 2.8 MB 74 kB/s
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Installing collected packages: sphinx-rtd-theme
Successfully installed sphinx-rtd-theme-0.5.1

基于python的量子线路打印小项目

我们先看一下使用的方法以及效果，再回过头来分析代码实现的原理：

if __name__ == '__main__':
    qc = QuantumCircuit(3)
    qc.apply(0, 0)
    qc.apply(1, (1,0))
    qc.apply(1, (2,1))
    qc.apply(1, (2,0))
    qc.apply(2)
    print (qc)

输出结果如下所示(注意使用等宽字体)：

 |0>     |0>     |0>
  |       |       |
  |       |       h
  |       |       |
  |       x ----- c
  |       |       |
  x ----- c       |
  |       |       |
  x ----- | ----- c
  |       |       |
  m       m       m

这里我们就可以了解到，这个项目的核心功能就是通过构造一个量子线路的对象，并且对这个对象逐一的施加量子门操作，就可以输出成对应的量子线路字符串。接下来我们再看看源码实现：

# qcprinter.py
"""
The module used for print a quantum circuit in string format.
"""

class QuantumCircuit:
    """The class of quantum circuit from input quantum logical gates.

    Parameters
        ----------
        qubits : int
            Totcal number of qubits used in this quantum circuit.

    Returns
        -------
        str
            The string format of quantum circuit.

    How to use?
        -------

        >>> qc = QuantumCircuit(3)
        >>> qc.apply(0, 0)
        >>> qc.apply(1, (1,0))
        >>> qc.apply(1, (2,1))
        >>> qc.apply(1, (2,0))
        >>> qc.apply(2)
        >>> print (qc)
    Example output
        -------

        >>>  |0>     |0>     |0>
        >>>   |       |       |
        >>>   |       |       h
        >>>   |       |       |
        >>>   |       x ----- c
        >>>   |       |       |
        >>>   x ----- c       |
        >>>   |       |       |
        >>>   x ----- | ----- c
        >>>   |       |       |
        >>>   m       m       m
    """
    def __init__(self, qubits=0):
        self.qubits = qubits
        self.str_circuit = ' |0>\t' * self.qubits + ' \n'

    def __str__(self):
        return self.str_circuit

    def apply(self, gate_type, qubit_index=0):
        """The function used to apply quantum logical gates on quantum state.

        Parameters
            ----------
            gate_type : int
                An index number represents for different type of quantum logical gates. 0 for h gate,
                1 for cx gate and 2 for measure gate.
            qubit_index: int, tuple, none
                The qubit index quantum logical gates allpied on. If the gate_type is h, the qubit_index
                would be int format. If the gate_type is cx, the qubit_index should be a tuple. If the
                gate_type is m, that means do measurement to all qubits, thus you do not need to set
                the value of qubit_index.

        Returns
            -------
            str
                The string format of quantum circuit.
        """
        if gate_type == 0:
            self.str_circuit += '  | \t' * self.qubits + '\n'
            self.str_circuit += '  | \t' * (self.qubits - qubit_index - 1) + '  h \t' + '  | \t' * (qubit_index) + '\n'
        elif gate_type == 1:
            self.str_circuit += '  | \t' * self.qubits + '\n'
            self.str_circuit += '  | \t' * (self.qubits - qubit_index[0] - 1) + '  x'
            for i in range(qubit_index[0] - qubit_index[1] - 1):
                self.str_circuit += ' ' + '-' * 5 + ' |'
            self.str_circuit +=  ' ' + '-' * 5 + ' c \t'
            self.str_circuit += '  | \t' * (qubit_index[1]) + '\n'
        elif gate_type == 2:
            self.str_circuit += '  | \t' * self.qubits + '\n'
            self.str_circuit += '  m \t' * self.qubits + '\n'
        else:
            pass

在这个简单的代码实现中，我们主要是为量子线路的类设置了一个魔法函数__str__，将我们所需要的量子线路的字符串作为整个对象的字符串返回值(关于python的魔法函数的使用方法可以参考前面这篇博客介绍)。这个字符串的类的核心功能是通过字符串拼接来实现的，我们不过多的赘述。这里为了方便操作，我们将量子逻辑门操作使用整型数字来替代：0代表\(H\)门，1代表\(CNOT\)门，2代表全局的量子测量操作。同时，为了展示API文档的制作过程，这里我们在类与函数内都写了一部分的示例注释代码，在下一个章节介绍一下文档的效果。

sphinx文档生成与效果一览

首先使用sphinx-quickstart来生成一些配置文件：

[dechin@dechin-manjaro circuit]$ sphinx-quickstart
欢迎使用 Sphinx 3.5.1 快速配置工具。

Please enter values for the following settings (just press Enter to
accept a default value, if one is given in brackets).

Selected root path: .

You have two options for placing the build directory for Sphinx output.
Either, you use a directory "_build" within the root path, or you separate
"source" and "build" directories within the root path.
> 独立的源文件和构建目录（y/n） [n]: y

The project name will occur in several places in the built documentation.
> 项目名称: qcprinter
> 作者名称: DechinPhy
> 项目发行版本 []: 1.0

If the documents are to be written in a language other than English,
you can select a language here by its language code. Sphinx will then
translate text that it generates into that language.

For a list of supported codes, see
https://www.sphinx-doc.org/en/master/usage/configuration.html#confval-language.
> 项目语种 [en]: en

创建文件 /home/dechin/projects/2021-quantum/circuit/source/conf.py。
创建文件 /home/dechin/projects/2021-quantum/circuit/source/index.rst。
创建文件 /home/dechin/projects/2021-quantum/circuit/Makefile。
创建文件 /home/dechin/projects/2021-quantum/circuit/make.bat。

完成：已创建初始目录结构。

You should now populate your master file /home/dechin/projects/2021-quantum/circuit/source/index.rst and create other documentation
source files. Use the Makefile to build the docs, like so:
   make builder
where "builder" is one of the supported builders, e.g. html, latex or linkcheck.

在这一系列的配置之后，会在当前目录下生成几个新的文件(由于我们选择了build和source分离的模式，因此这里会有2个目录)：

[dechin@dechin-manjaro circuit]$ ll
总用量 52
drwxr-xr-x 2 dechin dechin  4096  2月 23 00:04 build
-rw-r--r-- 1 dechin dechin   799  2月 23 00:04 make.bat
-rw-r--r-- 1 dechin dechin   638  2月 23 00:04 Makefile
-rw-r--r-- 1 dechin dechin  3055  2月 22 23:26 qcprinter.py
drwxr-xr-x 4 dechin dechin  4096  2月 23 00:04 source

在source目录下会产生一些配置文件，这里我们需要修改其中的conf.py文件：

[dechin@dechin-manjaro circuit]$ cd source/
[dechin@dechin-manjaro source]$ ll
总用量 16
-rw-r--r-- 1 dechin dechin 1884  2月 23 00:04 conf.py
-rw-r--r-- 1 dechin dechin  443  2月 23 00:04 index.rst
drwxr-xr-x 2 dechin dechin 4096  2月 23 00:04 _static
drwxr-xr-x 2 dechin dechin 4096  2月 23 00:04 _templates

可以参考楼主的配置方案，这里我们主要是将主题配置成了rtd的格式，同时打开了autodoc的选项以及通过sys配置了索引目录(索引目录不配置的话，有可能导致找不到模块，从而无法正常的生成API接口文档)：

[dechin@dechin-20n2s01200 circuit]$ cat source/conf.py
# Configuration file for the Sphinx documentation builder.
#
# This file only contains a selection of the most common options. For a full
# list see the documentation:
# https://www.sphinx-doc.org/en/master/usage/configuration.html

# -- Path setup --------------------------------------------------------------

# If extensions (or modules to document with autodoc) are in another directory,
# add these directories to sys.path here. If the directory is relative to the
# documentation root, use os.path.abspath to make it absolute, like shown here.
#
import os
import sys
sys.path.insert(0, os.path.abspath('..'))

# -- Project information -----------------------------------------------------

project = 'qcprinter'
copyright = '2021, DechinPhy'
author = 'DechinPhy'

# The full version, including alpha/beta/rc tags
release = '1.0'

# -- General configuration ---------------------------------------------------
import sphinx_rtd_theme
# Add any Sphinx extension module names here, as strings. They can be
# extensions coming with Sphinx (named 'sphinx.ext.*') or your custom
# ones.
extensions = ["sphinx_rtd_theme", "sphinx.ext.autodoc", "sphinx.ext.autosummary"
]

# Add any paths that contain templates here, relative to this directory.
templates_path = ['_templates']

# List of patterns, relative to source directory, that match files and
# directories to ignore when looking for source files.
# This pattern also affects html_static_path and html_extra_path.
exclude_patterns = []

# -- Options for HTML output -------------------------------------------------

# The theme to use for HTML and HTML Help pages.  See the documentation for
# a list of builtin themes.
#
html_theme = 'sphinx_rtd_theme'

# Add any paths that contain custom static files (such as style sheets) here,
# relative to this directory. They are copied after the builtin static files,
# so a file named "default.css" will overwrite the builtin "default.css".
html_static_path = ['_static']

执行sphinx-build指令将source中的rst文档编译成html文档，并输出到build目录下：

[dechin@dechin-manjaro circuit]$ sphinx-build source/ build/
正在运行 Sphinx v3.5.1
加载 pickled环境... 完成
构建 [mo]： 0 个 po 文件的目标文件已过期
构建 [html]： 0 个源文件的目标文件已过期
更新环境: 已添加 0，1 已更改，0 已移除
阅读源... [100%] qcprinter
/home/dechin/projects/2021-quantum/circuit/qcprinter.py:docstring of qcprinter.QuantumCircuit:4: WARNING: Unexpected section title or transition.

----------
/home/dechin/projects/2021-quantum/circuit/qcprinter.py:docstring of qcprinter.QuantumCircuit:9: WARNING: Unexpected section title or transition.

-------
/home/dechin/projects/2021-quantum/circuit/qcprinter.py:docstring of qcprinter.QuantumCircuit:14: WARNING: Unexpected section title or transition.

-------
/home/dechin/projects/2021-quantum/circuit/qcprinter.py:docstring of qcprinter.QuantumCircuit:24: WARNING: Unexpected section title or transition.

-------
/home/dechin/projects/2021-quantum/circuit/qcprinter.py:docstring of qcprinter.QuantumCircuit.apply:4: WARNING: Unexpected section title or transition.

----------
/home/dechin/projects/2021-quantum/circuit/qcprinter.py:docstring of qcprinter.QuantumCircuit.apply:15: WARNING: Unexpected section title or transition.

-------
查找当前已过期的文件... 没有找到
pickling环境... 完成
检查一致性... /home/dechin/projects/2021-quantum/circuit/source/modules.rst: WARNING: 文档没有加入到任何目录树中
完成
准备文件... 完成
写入输出... [ 33%] index                                                                                                                                                                                                                    写入输出... [ 66%] modules                                                                                                                                                                                                                  写入输出... [100%] qcprinter
generating indices... genindex py-modindex 完成
writing additional pages... search 完成
copying static files... 完成
copying extra files... 完成
dumping search index in English (code: en)... 完成
dumping object inventory... 完成
build 成功, 7 warnings.

HTML 页面保存在 build 目录。

在这个执行的过程中，有一部分的告警是跟注释规范相关的，其实不用处理也没有关系。接下来就可以打开build目录，查看已经生成成功的html文档，首先我们可以打开index.html，是常用的网页主页索引，大概内容如下图所示：



我们先点击这里的index和module看看内容，分别为下列的两个图所示：





最后在这个索引列表中我们点击进入qcprinter这个类中，去查看详细的类的文档说明：



相应的函数注释内容也会在接口文档中体现：



需要注意的是，如果相关的类或者函数是受保护的类型，那么在sphinx生成的文档中是不会显示的(构造过程中自动忽略)。

总结概要

在这篇文章中，我们主要通过一个量子线路打印的python项目介绍，也顺带通过sphinx将python项目的注释文档自动化的生成API接口文档，完成了一个项目开发及文档输出流程的简要分析，在实战中掌握更多的工具使用方法。

版权声明

本文首发链接为：https://www.cnblogs.com/dechinphy/p/qcprinter.html

作者ID：DechinPhy

更多原著文章请参考：https://www.cnblogs.com/dechinphy/