Python学习笔记之入门
函数
前面简单的就不说了，先记录一下最简单的数学函数：
| Operator | Name | Description |
|---|---|---|
a + b | 加 | a, b相加 |
a - b | 减 | a减b |
a * b | 乘 | a，b相乘 |
a / b | 除 | a除以b |
a // b | 整除 | a除以b，保留整数 |
a % b | 余数 | a除以b，余下的数 |
a ** b | 乘方 | a的b次方 |
-a | 负数 | 负 a |
自定义函数：
def least_difference(a, b, c): diff1 = abs(a - b) diff2 = abs(b - c) diff3 = abs(a - c) return min(diff1, diff2, diff3)这样就创建了一个名为“least_difference”的函数，它有三个变量“a, b, c”，其中“def”是声明函数的关键词，“：”后面的内容是当使用函数时执行的内容，“return”是函数的终止并输出右边的内容。
以一个具体包含数字的函数为例：
print( least_difference(1, 10, 100), least_difference(1, 10, 10), least_difference(5, 6, 7), # Python allows trailing commas in argument lists. How nice is that?)
输出结果：9 0 1当函数中没有“return”时：
def least_difference(a, b, c): """Return the smallest difference between any two numbers among a, b and c. """ diff1 = abs(a - b) diff2 = abs(b - c) diff3 = abs(a - c) min(diff1, diff2, diff3) print( least_difference(1, 10, 100), least_difference(1, 10, 10), least_difference(5, 6, 7),)
输出结果：None None None当不声明“return”时，最后的返回结果是“None”，结果没有意义。
round函数的妙用：
round函数一般用于保留有效数字，当round为正数，保留小数点后，round为负数，向前取整
num = 5743.141592654round(num,2)5743.14
num = 5743.141592654round(num,-2)5700.0
num = 5753.141592654round(num,-2)5800.0如上示例，第一个round取了两位有效数字，小数点后保留2位，第二个round参数为负数，向前取整两位，取整都是四舍五入，如后两个所示，第三个5753进行了五入，而第二个是四舍。
条件
Booleans
Python 有一种类型 bool 包含两个值: True 和 False.
x = Trueprint(x)print(type(x))输出：True<class 'bool'>相比直接将 True 或 False 放到代码中， 我们一般通过 布尔运算 获得布尔值。 这些运算可以用来回答 yes/no 的问题.。以下是一些布尔运算符号。
Comparison Operations
| Operation | Description | Operation | Description | |
|---|---|---|---|---|
a == b | a 等于 b | a != b | a 不等于 b | |
a < b | a 小于 b | a > b | a 大于 b | |
a <= b | a 小于等于 b | a >= b | a 大于等于 b |
def can_run_for_president(age): """Can someone of the given age run for president in the US?""" # The US Constitution says you must "have attained to the Age of thirty-five Years" return age >= 35
print("Can a 19-year-old run for president?", can_run_for_president(19))print("Can a 45-year-old run for president?", can_run_for_president(45))#输出结果：Can a 19-year-old run for president? FalseCan a 45-year-old run for president? TrueCombining Boolean Values
Python 提供合并布尔运算符 "且", "并", and "非"。对应的 Python 运算符是: and, or, and not.
def can_run_for_president(age, is_natural_born_citizen): """Can someone of the given age and citizenship status run for president in the US?""" # The US Constitution says you must be a natural born citizen *and* at least 35 years old return is_natural_born_citizen and (age >= 35)
print(can_run_for_president(19, True))print(can_run_for_president(55, False))print(can_run_for_president(55, True))Boolean conversion
我们知道 int(), 可以把值转换为整形, 以及 float(), 可以转换为浮点型, 同样 Python有一个 bool() 函数可以把其他值转换为布尔值。
print(bool(1)) # all numbers are treated as true, except 0print(bool(0))print(bool("asf")) # all strings are treated as true, except the empty string ""print(bool(""))# Generally empty sequences (strings, lists, and other types we've yet to see like lists and tuples)# are "falsey" and the rest are "truthy"输出结果：TrueFalseTrueFalse列表
Lists 在 Python 代表了一系列有序的值:
planets = ['Mercury', 'Venus', 'Earth', 'Mars', 'Jupiter', 'Saturn', 'Uranus', 'Neptune']Indexing索引
你可以使用方括号[ ]来索引到列表中单独的元素。
哪个星球距离太阳最近? Python 索引基于0， 因此第一个元素的索引为0.
planets[0]'Mercury'planets[1]'Venus'最远的是哪个星球呢？python倒数使用-1
planets[-1]'NeptuneSlicing切片
前三个星球是哪些? 我们可以使用切片slicing来回答这个问题:
planets[0:3]['Mercury', 'Venus', 'Earth']planets[:3]['Mercury', 'Venus', 'Earth']planets[3:] #从索引3到最后['Mars', 'Jupiter', 'Saturn', 'Uranus', 'Neptune']planets[1:-1] #去除首末剩余的元素['Venus', 'Earth', 'Mars', 'Jupiter', 'Saturn', 'Uranus'planets[-3:] #最后三个元素['Saturn', 'Uranus', 'Neptune']Changing lists
列表是可以变的，我们可以按位置修改列表，如下，我们把“Mars”进行修改：
planets[3] = 'Malacandra'planets['Mercury', 'Venus', 'Earth', 'Malacandra', 'Jupiter', 'Saturn', 'Uranus', 'Neptune']List functions列表函数
len 说明一个列表的长度:
# How many planets are there?len(planets)8sorted生成一个排序后的列表:
# The planets sorted in alphabetical order根据字母顺序排序sorted(planets)['Earth', 'Jupiter', 'Mars', 'Mercury', 'Neptune', 'Saturn', 'Uranus', 'Venus']