python简易计算器

Question

一段用python写的简易计算器代码，然后对于其中一些代码的用处不是很理解，
请求协助解释一下，提问都在注释里

import tkinter as tk

ERROR = "Error!"

def string_to_double(s):
    try:
        val = float(s);#display_str.get()得到的是字符串，然后float(s)就可以直接转化成float type了？
        return val;
    except ValueError:#这里用except是不是有点多余，一般按的都是数字，也没法按字符串
        return None

def clear_if_error():
    s = display_str.get()
    if s == ERROR:
        display_str.set("0")

def double_to_string(v):
    s = str(v)
    if 'e' in s:
        return ERROR
    if '.' in s:
        while s[-1] == '0':
            s = s[0:-1]
    while len(s) > 10 and s[-1] != '.':#如果len(s)>10了，岂不是要出错了？这里用while的意义是什么
        s = s[0:-1]
    if s[-1] == '.':
        s = s[0:-1]
    if len(s) > 10:
        return ERROR
    return s

def do_clear():
    global accumulator, last_operation
    display_str.set("0")
    accumulator = 0.
    last_operation = ""


def do_digit_0():
    clear_if_error()
    s = display_str.get()
    if len(s) < 10 and s != "0":
        display_str.set(s + '0')


def do_digit_x(dig):
    clear_if_error()
    s = display_str.get()
    if len(s) < 10:
        if s == "0":
            display_str.set(dig)
        else:
            display_str.set(s + dig)


def do_dot():
    clear_if_error()
    s = display_str.get()
    if len(s) < 10 and "." not in s:
        display_str.set(s + ".")


def do_plus():
    global last_operation, accumulator
    clear_if_error()
    last_operation = "+"
    accumulator = string_to_double(display_str.get())
    display_str.set("0")


def do_minus():
    global last_operation, accumulator
    clear_if_error()
    last_operation = "-"
    accumulator = string_to_double(display_str.get())
    display_str.set("0")


def do_mult():
    global last_operation, accumulator
    clear_if_error()
    last_operation = "*"
    accumulator = string_to_double(display_str.get())
    display_str.set("0")


def do_divd():
    global last_operation, accumulator
    clear_if_error()
    last_operation = "/"
    accumulator = string_to_double(display_str.get())
    display_str.set("0")


def do_equal():
    global last_operation, accumulator
    clear_if_error()
    value = string_to_double(display_str.get())
    if last_operation == "+":
        accumulator += value#举个例子，如果运行3+4，是不是accumulator=3,value=4,然后accumulator=7,为什么accumulator会认定是3，value会认定是4？
    elif last_operation == "-":
        accumulator -= value
    elif last_operation == "*":
        accumulator *= value
    elif last_operation == "/":
        if value != 0:
            accumulator /= value
        else:
            display_str.set(ERROR);
            return;
    display_str.set(double_to_string(accumulator))#根据计算器运行，如果我按一个数字，譬如说7，然后直接按等号"=",然后entry里显示0，但在do_equal()这个函数里并没有表示有这么一个功能


def do_plusminus():
    clear_if_error()
    display_str.set(double_to_string(-string_to_double(display_str.get())))


window = tk.Tk()
window.title('Calc')
display_str = tk.StringVar()
display_str.set("0")
stick = tk.N + tk.S + tk.E + tk.W#这个上北下南左西右东的格局是用加号做布置的？
display = tk.Entry(window, width=10, font=("Courier New", "15", "bold"), textvariable=display_str, justify=tk.RIGHT)
display.grid(row=0, columnspan=5, sticky=stick)
digit7 = tk.Button(window, text="7", command=lambda: do_digit_x("7"))#为什么这里需要用到lambda？
digit7.grid(row=1, column=0, sticky=stick)
digit8 = tk.Button(window, text="8", command=lambda: do_digit_x("8"))
digit8.grid(row=1, column=1, sticky=stick)
digit9 = tk.Button(window, text="9", command=lambda: do_digit_x("9"))
digit9.grid(row=1, column=2, sticky=stick)
plus = tk.Button(window, text="+", command=do_plus)
plus.grid(row=1, column=4, sticky=stick)
digit4 = tk.Button(window, text="4", command=lambda: do_digit_x("4"))
digit4.grid(row=2, column=0, sticky=stick)
digit5 = tk.Button(window, text="5", command=lambda: do_digit_x("5"))
digit5.grid(row=2, column=1, sticky=stick)
digit6 = tk.Button(window, text="6", command=lambda: do_digit_x("6"))
digit6.grid(row=2, column=2, sticky=stick)
minus = tk.Button(window, text="-", command=do_minus)
minus.grid(row=2, column=4, sticky=stick)
digit1 = tk.Button(window, text="1", command=lambda: do_digit_x("1"))
digit1.grid(row=3, column=0, sticky=stick)
digit2 = tk.Button(window, text="2", command=lambda: do_digit_x("2"))
digit2.grid(row=3, column=1, sticky=stick)
digit3 = tk.Button(window, text="3", command=lambda: do_digit_x("3"))
digit3.grid(row=3, column=2, sticky=stick)
equal = tk.Button(window, text=" = ", command=do_equal)
equal.grid(row=3, column=3, sticky=stick)
mult = tk.Button(window, text="*", command=do_mult)
mult.grid(row=3, column=4, sticky=stick)
digit0 = tk.Button(window, text="0", command=do_digit_0)
digit0.grid(row=4, column=0, sticky=stick)
dot = tk.Button(window, text=" . ", command=do_dot)
dot.grid(row=4, column=2, sticky=stick)
clear = tk.Button(window, text="C", command=do_clear)
clear.grid(row=4, column=1, sticky=stick)
plusminus = tk.Button(window, text="+/-", command=do_plusminus)
plusminus.grid(row=4, column=3, sticky=stick)
divd = tk.Button(window, text="/", command=do_divd)
divd.grid(row=4, column=4, sticky=stick)
accumulator = 0.
last_operation = ""
window.mainloop()

Answer 1

注释大概这样

def string_to_double(s):
    try:
        val = float(s);#类型转换是这样的，转成int型则是 int(s)
        return val;
    #就目前这个代码通过按钮输入可以不需要，但是为了程序的健壮性还是加上比较好，防止逻辑没处理好传入其他非数字内容导致程序崩溃
    #也许到时整个工具库，用这个函数来转的话没发控制输入来源这样写就比较好
    except ValueError:
        return None


def double_to_string(v):
    s = str(v)
    if 'e' in s:
        return ERROR
    if '.' in s:
        while s[-1] == '0':
            s = s[0:-1]

    #这块代码控制转换成字符串后，字符串最大长度为10，超过就删除最后一个数字直到长度为10
    #这个处理还是有bug，如果是123456789011.1，则处理后变为123456789011.,长度还是12超了，所以最后的len(s)>10返回了ERROR
    while len(s) > 10 and s[-1] != '.':#如果len(s)>10了，岂不是要出错了？这里用while的意义是什么
        s = s[0:-1]
        print(1,s)
    if s[-1] == '.':
        s = s[0:-1]
    if len(s) > 10:
        return ERROR
    return s


def do_equal():
    global last_operation, accumulator
    clear_if_error()
    value = string_to_double(display_str.get())

    if last_operation == "+":
        #accumulator是按下操作符前输入的数字，3+4，所以accumulator=3，value=4，计算后将结果赋值给accumulator
        #accumulator += value实际就是accumulator = accumulator+value，只是简写了
        accumulator += value#举个例子，如果运行3+4，是不是accumulator=3,value=4,然后accumulator=7,为什么accumulator会认定是3，value会认定是4？
    elif last_operation == "-":
        accumulator -= value
    elif last_operation == "*":
        accumulator *= value
    elif last_operation == "/":
        if value != 0:
            accumulator /= value
        else:
            display_str.set(ERROR);
            return;
    #因为最下面accumulator默认为0，刚开始按下数字后，last_operation为空，上面的操作都没执行，这里直接就是将0转数字后赋值给输入框绑定的display_str变量了
    display_str.set(double_to_string(accumulator))



window = tk.Tk()
window.title('Calc')
display_str = tk.StringVar()
display_str.set("0")
stick = tk.N + tk.S + tk.E + tk.W#是的N/S/E/W，分别代表上对齐/下对齐/左对齐/右对齐，多个方向对齐用+链接，ticky=N+S，意为顶端和低端都要对齐
display = tk.Entry(window, width=10, font=("Courier New", "15", "bold"), textvariable=display_str, justify=tk.RIGHT)
display.grid(row=0, columnspan=5, sticky=stick)
digit7 = tk.Button(window, text="7", command=lambda: do_digit_x("7"))#command需要无参函数，要传递参数需要lambda。或者按照上个例子那样用Bind来绑定事件，通过事件获取按钮显示的text也行