Python

基本语法与函数

• 赋值语句都是先算右值再依序与左侧名称绑定

  >>> x,y=3,4.5
      x,y=y,x
  

• print返回值始终为None

  >>> print (print(1),print(2))
  1
  2
  None None
  

• 函数及形参名称选取：

  1. 单词之间用下划线
  2. 函数名反映对参数的操作或结果
  3. 参数名反映其作用

• 函数是一种抽象:domain(接受的参数合集) → range(返回的参数合集) intent(计算输入和输出的关系以及可能产生的副作用)

• /就正常除法，得到浮点数; //向下取整(类似C中int型的/)

条件控制 高阶函数 环境

• 好的函数是抽象的：通用、简单、不重复
• Python是以缩进划分语块的 (《繁星·春水》)
• Boolean Operators:and or not & short-circuiting:

  >>> -1 and 2 and 0 and 1
  0
  >>> False or 3124 or 1/0
  3124 
  

  换言之，从左向右，某位置的表达式已经可以保证整个语句的真假可判断时，计算结果为此表达式的结果，否则为最右侧值
• 执行比较并返回布尔值的函数通常is开头且无下划线，如isinstance isfinite
• Nested function definition: Python中函数可以嵌套定义
• Curring(柯里化):使用高阶函数将多元函数转化为只接受一个参数的函数链

  >>> def curried_pow(x):
              def h(y):
                  return pow(x,y)
              return h
  

• Lambda expression 是函数的简化，但可读性差

Note

lambda x : f(g(x))

A function takes x and returns f(g(x))

others

ifelse利用短路

例如：

abs_number = lambda x : x if x >= 0 else -x

是取x绝对值（有点像C里的?）

源自于hw02的mod_maker,利用or和 and的短路问题避免用ifelse而控制条件输出

>>> def mod_maker():
        return lambda x,y : x%y or True
    mod = mod_maker()
    '''
    >>>mod(4,8)
    4
    >>>mod(8,4)
    True
    '''

• 函数装饰器（decorator）

example

>>> def trace(fn):
        def wrapped(x):
            print('-> ', fn, '(', x, ')')
            return fn(x)
        return wrapped

>>> @trace
    def triple(x):
        return 3 * x

>>> triple(12)
->  <function triple at 0x102a39848> ( 12 )
36

函数应用 + 利用构建函数最终完成一段音频的制作

Eg

    # Example: Sound

from wave import open
#struct--encoding integers in the format that WAV files require
from struct import Struct
from math import floor

frame_rate = 11025

def encode(x):
    """Encode float x between -1 and 1 as two bytes.
    (See https://docs.python.org/3/library/struct.html)
    """
    i = int(16384 * x)
    return Struct('h').pack(i)

def play(sampler, name='song.wav', seconds=2):
    """Write the output of a sampler function as a wav file.
    (See https://docs.python.org/3/library/wave.html)
    """
    out = open(name, 'wb')
    out.setnchannels(1)
    out.setsampwidth(2)
    out.setframerate(frame_rate)
    t = 0
    while t < seconds * frame_rate:
        sample = sampler(t)
        out.writeframes(encode(sample))
        t = t + 1
    out.close()

def tri(frequency, amplitude=0.3):
    """A continuous triangle wave."""
    period = frame_rate // frequency
    def sampler(t):
        saw_wave = t / period - floor(t / period + 0.5)
        tri_wave = 2 * abs(2 * saw_wave) - 1
        return amplitude * tri_wave
    return sampler

c_freq, e_freq, g_freq = 261.63, 329.63, 392.00

play(tri(e_freq))

def note(f, start, end, fade=.01):
    """Play f for a fixed duration."""
    def sampler(t):
        seconds = t / frame_rate
        if seconds < start:
            return 0
        elif seconds > end:
            return 0
        elif seconds < start + fade:
            return (seconds - start) / fade * f(t)
        elif seconds > end - fade:
            return (end - seconds) / fade * f(t)
        else:
            return f(t)
    return sampler

play(note(tri(e_freq), 1, 1.5))

def both(f, g):
    return lambda t: f(t) + g(t)

c = tri(c_freq)
e = tri(e_freq)
g = tri(g_freq)
low_g = tri(g_freq / 2)

play(both(note(e, 0, 1/8), note(low_g, 1/8, 3/8)))

play(both(note(c, 0, 1), both(note(e, 0, 1), note(g, 0, 1))))

def mario(c, e, g, low_g):
    z = 0
    song = note(e, z, z + 1/8)
    z += 1/8
    song = both(song, note(e, z, z + 1/8))
    z += 1/4
    song = both(song, note(e, z, z + 1/8))
    z += 1/4
    song = both(song, note(c, z, z + 1/8))
    z += 1/8
    song = both(song, note(e, z, z + 1/8))
    z += 1/4
    song = both(song, note(g, z, z + 1/4))
    z += 1/2
    song = both(song, note(low_g, z, z + 1/4))
    return song

def mario_at(octave):
    c = tri(octave * c_freq)
    e = tri(octave * e_freq)
    g = tri(octave * g_freq)
    low_g = tri(octave * g_freq / 2)
    return mario(c, e, g, low_g)
play(mario_at(1/2))

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最后更新: 2024年7月6日 01:24:24
创建日期: 2024年3月9日 08:53:42