第四讲 Python 模块 DONE

1. 准备
2. 程序的测试
- 2.1. 接口测试
3. Python 模块
- 3.1. 多层模块的加载
4. 文件读取

1 准备

安装 wget
```
apt install wget
```

2 程序的测试

在撰写较长较复杂的程序时，我最关心的问题是，到底如何保证程序正确呢？眼睛一直盯着它，多看几遍就对了吗？不是。另一个相关的问题是，我已经写出一个正确的程序，但我修改完善它，如果保证从改动前后程序的逻辑不变呢？否则，如果一不小心改坏就闯祸了，还不如不完善。如何快速确认完善后的程序是好的？一个程序的中间结果应该当被人理解，是“透明”原则的要求，可以通过调试来确保程序的正确性。但是问题远没有这么简单，如果我们每次都使用程序调试方法如 print breakpoint 等检查中间结果，势必会让保证程序正确性的工作变得重复乏味，成为自己的负担。 “一次”原则应用到这里，要求我们写一个自动的测试程序，判断主程序的正确性。每次主程序有修改，都自动运行测试程序，确认原有功能是否正确。有时甚至我们在写主程序之前，应当先把测试程序写好，给定特定的输入输出，用测试程序来定义主程序的功能。当主程序需要填加新功能时，也是首先扩展测试程序，再写主程序。这种开发的方式，叫做“测试驱动的研发”。在团队分工中，可以一个人写测试程序，另一个人写主程序，实现分工。测试分为单元测试，用于确保某个函数的正确性，以及集成测试，用于保证程序总体（特别是各函数之间的接口）达到设计要求。研发的过程中，测试与主程序相辅相成，相互依赖着前进。自动测试是有效避免“祖传代码”的途径。“祖传代码”意为一个长久使用但疏于维护的程序，因为依赖旧环境执行，兼容性极差。但是没有人敢于完善它，因为修改使程序损坏的代价远高于兼容性差带来的不便，随着时间的推移，程序变得越来越不好用，也越来越不可能被修改。破局的关键是测试程序，首先要通过测试来定义清楚，什么样的结果是“改得好”，什么样是“改坏了”。随后的主体完善就有了客观标准，得以顺利进行。测试和主程序像两条腿，交替前进。缺少了测试程序，一条腿无法走路，才变成了“祖传程序”。

2.1 接口测试

接口的定义在大规模的项目极其重要，相比之下程序的具体实现甚至不那么核心了。在练习中，一定要注意输入与输出的格式定义，不能有任何差池。透明原则中，人类理解中间结果的大前提，是机器程序的输入输出符合约定的规范。这样规范，有些不近人情，看起来一样的 “3.500000000”， “3.5e0” ， “3.4999999999” 等，如果在规范之外，就可能造成后续程序的异常。在复杂的大项目里，测试是解耦合的重要方法。软件工程的建议方案是，程序是否正常运行，满足输入输出要求，主要靠测试程序来对每个主程序进行单元测试，模拟上游的合法输入，模拟下游的程序验证输出的合理性。这些测试尽可能是全自动的，有助于提升团队整体的效率。现实世界里，在合法的范围内，输入数据也可能是千差万别。设计程序中，要考虑这种兼容性和灵活处理的空间。测试程序更应当从多种极端情形去构造测例，推动程序的兼容性。

3 Python 模块

Python 的模块是把函数等聚集起来的名字空间，由目录或者文件划定。它既可以由 Python 实现，也可以由 C 和 Fortran 等编译语言实现。半个多世纪积累下来的优秀程序，大多可以以 Python 模块的形式出现，被复用，实践“一次”原则。用编译语言实现 Python 的模块，效率可以比纯 Python 的高，代价是撰写代码的难度增加，尤其在无既有代码时。但有些时候，程序效率非常重要，可以考虑用 C 或 Fortran 重新实现最影响性能的 Python 模块。

Python 的模块都具有详实的在线帮助。

import math
help(math)

Help on module math:

NAME
    math

MODULE REFERENCE
    https://docs.python.org/3.9/library/math

    The following documentation is automatically generated from the Python
    source files.  It may be incomplete, incorrect or include features that
    are considered implementation detail and may vary between Python
    implementations.  When in doubt, consult the module reference at the
    location listed above.

DESCRIPTION
    This module provides access to the mathematical functions
    defined by the C standard.

FUNCTIONS
    acos(x, /)
        Return the arc cosine (measured in radians) of x.

        The result is between 0 and pi.

    acosh(x, /)
        Return the inverse hyperbolic cosine of x.

    asin(x, /)
        Return the arc sine (measured in radians) of x.

        The result is between -pi/2 and pi/2.

    asinh(x, /)
        Return the inverse hyperbolic sine of x.

    atan(x, /)
        Return the arc tangent (measured in radians) of x.

        The result is between -pi/2 and pi/2.

    atan2(y, x, /)
        Return the arc tangent (measured in radians) of y/x.

        Unlike atan(y/x), the signs of both x and y are considered.

    atanh(x, /)
        Return the inverse hyperbolic tangent of x.

    ceil(x, /)
        Return the ceiling of x as an Integral.

        This is the smallest integer >= x.

    comb(n, k, /)
        Number of ways to choose k items from n items without repetition and without order.

        Evaluates to n! / (k! * (n - k)!) when k <= n and evaluates
        to zero when k > n.

        Also called the binomial coefficient because it is equivalent
        to the coefficient of k-th term in polynomial expansion of the
        expression (1 + x)**n.

        Raises TypeError if either of the arguments are not integers.
        Raises ValueError if either of the arguments are negative.

    copysign(x, y, /)
        Return a float with the magnitude (absolute value) of x but the sign of y.

        On platforms that support signed zeros, copysign(1.0, -0.0)
        returns -1.0.

    cos(x, /)
        Return the cosine of x (measured in radians).

    cosh(x, /)
        Return the hyperbolic cosine of x.

    degrees(x, /)
        Convert angle x from radians to degrees.

    dist(p, q, /)
        Return the Euclidean distance between two points p and q.

        The points should be specified as sequences (or iterables) of
        coordinates.  Both inputs must have the same dimension.

        Roughly equivalent to:
            sqrt(sum((px - qx) ** 2.0 for px, qx in zip(p, q)))

    erf(x, /)
        Error function at x.

    erfc(x, /)
        Complementary error function at x.

    exp(x, /)
        Return e raised to the power of x.

    expm1(x, /)
        Return exp(x)-1.

        This function avoids the loss of precision involved in the direct evaluation of exp(x)-1 for small x.

    fabs(x, /)
        Return the absolute value of the float x.

    factorial(x, /)
        Find x!.

        Raise a ValueError if x is negative or non-integral.

    floor(x, /)
        Return the floor of x as an Integral.

        This is the largest integer <= x.

    fmod(x, y, /)
        Return fmod(x, y), according to platform C.

        x % y may differ.

    frexp(x, /)
        Return the mantissa and exponent of x, as pair (m, e).

        m is a float and e is an int, such that x = m * 2.**e.
        If x is 0, m and e are both 0.  Else 0.5 <= abs(m) < 1.0.

    fsum(seq, /)
        Return an accurate floating point sum of values in the iterable seq.

        Assumes IEEE-754 floating point arithmetic.

    gamma(x, /)
        Gamma function at x.

    gcd(*integers)
        Greatest Common Divisor.

    hypot(...)
        hypot(*coordinates) -> value

        Multidimensional Euclidean distance from the origin to a point.

        Roughly equivalent to:
            sqrt(sum(x**2 for x in coordinates))

        For a two dimensional point (x, y), gives the hypotenuse
        using the Pythagorean theorem:  sqrt(x*x + y*y).

        For example, the hypotenuse of a 3/4/5 right triangle is:

            >>> hypot(3.0, 4.0)
            5.0

    isclose(a, b, *, rel_tol=1e-09, abs_tol=0.0)
        Determine whether two floating point numbers are close in value.

          rel_tol
            maximum difference for being considered "close", relative to the
            magnitude of the input values
          abs_tol
            maximum difference for being considered "close", regardless of the
            magnitude of the input values

        Return True if a is close in value to b, and False otherwise.

        For the values to be considered close, the difference between them
        must be smaller than at least one of the tolerances.

        -inf, inf and NaN behave similarly to the IEEE 754 Standard.  That
        is, NaN is not close to anything, even itself.  inf and -inf are
        only close to themselves.

    isfinite(x, /)
        Return True if x is neither an infinity nor a NaN, and False otherwise.

    isinf(x, /)
        Return True if x is a positive or negative infinity, and False otherwise.

    isnan(x, /)
        Return True if x is a NaN (not a number), and False otherwise.

    isqrt(n, /)
        Return the integer part of the square root of the input.

    lcm(*integers)
        Least Common Multiple.

    ldexp(x, i, /)
        Return x * (2**i).

        This is essentially the inverse of frexp().

    lgamma(x, /)
        Natural logarithm of absolute value of Gamma function at x.

    log(...)
        log(x, [base=math.e])
        Return the logarithm of x to the given base.

        If the base not specified, returns the natural logarithm (base e) of x.

    log10(x, /)
        Return the base 10 logarithm of x.

    log1p(x, /)
        Return the natural logarithm of 1+x (base e).

        The result is computed in a way which is accurate for x near zero.

    log2(x, /)
        Return the base 2 logarithm of x.

    modf(x, /)
        Return the fractional and integer parts of x.

        Both results carry the sign of x and are floats.

    nextafter(x, y, /)
        Return the next floating-point value after x towards y.

    perm(n, k=None, /)
        Number of ways to choose k items from n items without repetition and with order.

        Evaluates to n! / (n - k)! when k <= n and evaluates
        to zero when k > n.

        If k is not specified or is None, then k defaults to n
        and the function returns n!.

        Raises TypeError if either of the arguments are not integers.
        Raises ValueError if either of the arguments are negative.

    pow(x, y, /)
        Return x**y (x to the power of y).

    prod(iterable, /, *, start=1)
        Calculate the product of all the elements in the input iterable.

        The default start value for the product is 1.

        When the iterable is empty, return the start value.  This function is
        intended specifically for use with numeric values and may reject
        non-numeric types.

    radians(x, /)
        Convert angle x from degrees to radians.

    remainder(x, y, /)
        Difference between x and the closest integer multiple of y.

        Return x - n*y where n*y is the closest integer multiple of y.
        In the case where x is exactly halfway between two multiples of
        y, the nearest even value of n is used. The result is always exact.

    sin(x, /)
        Return the sine of x (measured in radians).

    sinh(x, /)
        Return the hyperbolic sine of x.

    sqrt(x, /)
        Return the square root of x.

    tan(x, /)
        Return the tangent of x (measured in radians).

    tanh(x, /)
        Return the hyperbolic tangent of x.

    trunc(x, /)
        Truncates the Real x to the nearest Integral toward 0.

        Uses the __trunc__ magic method.

    ulp(x, /)
        Return the value of the least significant bit of the float x.

DATA
    e = 2.718281828459045
    inf = inf
    nan = nan
    pi = 3.141592653589793
    tau = 6.283185307179586

FILE
    /usr/lib/python3.9/lib-dynload/math.cpython-39-x86_64-linux-gnu.so

在 REPL 环境中，我们还可以输入 math. 后，按 TAB 建来给出后面可能接的函数，非常有助于去探索一个新的模块。看到不明白含义的 TAB 提示，配合 help 即可迅速掌握。

help(math.gcd)

Help on built-in function gcd in module math:

gcd(*integers)
    Greatest Common Divisor.

可发现性，是 Python 易于入门的重要特征。

装载模块时，能够自定义名称，缩短程序的长度来增强可读性。

import math as m
m.factorial(10)

3.1 多层模块的加载

当模块中的内容很多时，会被安排在不同层次的名字空间中。有多种等价的方法访问它们：

import os
from os.path import abspath
from os.path import abspath as absp
abspath is os.path.abspath, abspath is absp

(True, True)

os.path.abspath 是 os 模块的 path 子模块中返回绝对路径的函数。直接使用很啰嗦，配合 from 会简洁很多。

4 文件读取

文本文件输入输出，可使用 open() 。用 for 循环可将打开的文件视为迭代器逐行读入。每次循环得到字符串可进一步处理。在 Python 当前文件夹的命令行中，下载一个文本文件样例，

wget --progress=dot 'http://hep.tsinghua.edu.cn/~orv/pd/iterator.txt'

--2022-07-07 10:22:24--  http://hep.tsinghua.edu.cn/~orv/pd/iterator.txt
Resolving hep.tsinghua.edu.cn... 101.6.6.219, 2402:f000:1:416:101:6:6:219
Connecting to hep.tsinghua.edu.cn|101.6.6.219|:80... connected.
HTTP request sent, awaiting response... 200 OK
Length: 268 [text/plain]
Saving to: ‘iterator.txt’

     0K                                                       100% 37.3M=0s

2022-07-07 10:22:24 (37.3 MB/s) - ‘iterator.txt’ saved [268/268]

使用 open() 读入，用迭代器逐行读取。

for l in open("iterator.txt"):
    print(l, end="")

Iterator Types

Python supports a concept of iteration over containers. This is
implemented using two distinct methods; these are used to allow
user-defined classes to support iteration. Sequences, described below
in more detail, always support the iteration methods.

读入的字符串带有换行符，与 print 叠加会有空行，因此给 print 传了 end 参数。

写入文本文件时，令 open() 以写模式打开。

f = open("log.txt", 'w')
f.write("第一天 概论\n")
f.write("第二天 Python 入门\n")
f.close()

"\n" 是换行符。在命令行中查看输出的文件，

cat log.txt

第一天 概论
第二天 Python 入门

有了输入输出的方法，我们可以把 Python 的字符串处理功能与程序结构、复合数据结构等结合起来，完成实用的文本处理工作。