阿里云Python课程笔记（一）基础易错点

a = "hello"b = "hello"print(a is b, a == b)  # True Trueprint(a is not b, a != b)  # False False

a = ["hello"]b = ["hello"]print(a is b, a == b)  # False Trueprint(a is not b, a != b)  # True False

print(-3 ** 2)  # -9print(3 ** -2)  # 0.1111111111111111print(1 3 + 2 & 7)  # 0print(-3 * 2 + 5 / -2 - 4)  # -12.5print(3 4 and 4 5)  # True

b = dir(int)print(b)
# ['__abs__', '__add__', '__and__', '__bool__', '__ceil__', '__class__',# '__delattr__', '__dir__', '__divmod__', '__doc__', '__eq__',# '__float__', '__floor__', '__floordiv__', '__format__', '__ge__',# '__getattribute__', '__getnewargs__', '__gt__', '__hash__',# '__index__', '__init__', '__init_subclass__', '__int__', '__invert__',# '__le__', '__lshift__', '__lt__', '__mod__', '__mul__', '__ne__',# '__neg__', '__new__', '__or__', '__pos__', '__pow__', '__radd__',# '__rand__', '__rdivmod__', '__reduce__', '__reduce_ex__', '__repr__',# '__rfloordiv__', '__rlshift__', '__rmod__', '__rmul__', '__ror__',# '__round__', '__rpow__', '__rrshift__', '__rshift__', '__rsub__',# '__rtruediv__', '__rxor__', '__setattr__', '__sizeof__', '__str__',# '__sub__', '__subclasshook__', '__truediv__', '__trunc__', '__xor__',# 'bit_length', 'conjugate', 'denominator', 'from_bytes', 'imag',# 'numerator', 'real', 'to_bytes']

import decimalfrom decimal import Decimala = decimal.getcontext()print(a)

decimal.getcontext().prec = 4c = Decimal(1) / Decimal(3)print(c)#使1/3保留四位，用getcontext().prec来调整精度

print(isinstance(1, int))  # Trueprint(isinstance(5.2, float))  # Trueprint(isinstance(True, bool))  # Trueprint(isinstance('5.2', str))  # True

print(int('520'))  # 520print(int(520.52))  # 520print(float('520.52'))  # 520.52print(float(520))  # 520.0print(str(10 + 10))  # 20print(str(10.1 + 5.2))  # 15.3

print(*objects, sep=' ', end='n', file=sys.stdout, flush=False)

my_list = ['lsgogroup']my_list.pop(0)assert len(my_list) > 0
# AssertionError

for 迭代变量 in 可迭代对象:    代码块    for每次循环，迭代变量被设置为可迭代对象的当前元素，提供给代码块使用。    member = ['张三', '李四', '刘德华', '刘六', '周润发']for each in member:    print(each)
# 张三# 李四# 刘德华# 刘六# 周润发
for i in range(len(member)):    print(member[i])
# 张三# 李四# 刘德华# 刘六# 周润发

dic = {'a': 1, 'b': 2, 'c': 3, 'd': 4}
for key, value in dic.items():    print(key, value, sep=':', end=' ')    # a:1 b:2 c:3 d:4

range([start,] stop[, step=1])

for i in range(1, 10, 2):    print(i)
# 1# 3# 5# 7# 9

enumerate(sequence, [start=0])

languages = ['Python', 'R', 'Matlab', 'C++']for language in languages:    print('I love', language)print('Done!')# I love Python# I love R# I love Matlab# I love C++# Done!for i, language in enumerate(languages, 2):    print(i, 'I love', language)print('Done!')# 2 I love Python# 3 I love R# 4 I love Matlab# 5 I love C++# Done!

def a_func():    pass

[ expr for value in collection [if condition] ]

x = [[i, j] for i in range(0, 3) for j in range(0, 3)]print(x)# [[0, 0], [0, 1], [0, 2], [1, 0], [1, 1], [1, 2], [2, 0], [2, 1], [2, 2]]x[0][0] = 10print(x)# [[10, 0], [0, 1], [0, 2], [1, 0], [1, 1], [1, 2], [2, 0], [2, 1], [2, 2]]

( expr for value in collection [if condition] )

a = (x for x in range(10))print(a)#  at 0x0000025BE511CC48>print(tuple(a))# (0, 1, 2, 3, 4, 5, 6, 7, 8, 9)

{ key_expr: value_expr for value in collection [if condition] }

b = {i: i % 2 == 0 for i in range(10) if i % 3 == 0}print(b)# {0: True, 3: False, 6: True, 9: False}

{ expr for value in collection [if condition] }

c = {i for i in [1, 2, 3, 4, 5, 5, 6, 4, 3, 2, 1]}print(c)# {1, 2, 3, 4, 5, 6}

try:    检测范围except Exception[as reason]:    出现异常后的处理代码

try:    int("abc")    s = 1 + '1'    f = open('test.txt')    print(f.read())    f.close()except OSError as error:    print('打开文件出错n原因是：' + str(error))except TypeError as error:    print('类型出错n原因是：' + str(error))except ValueError as error:    print('数值出错n原因是：' + str(error))
# 数值出错# 原因是：invalid literal for int() with base 10: 'abc'
try:    s = 1 + '1'    int("abc")    f = open('test.txt')    print(f.read())    f.close()except (OSError, TypeError, ValueError) as error:    print('出错了！n原因是：' + str(error))
# 出错了！# 原因是：unsupported operand type(s) for +: 'int' and 'str'

def divide(x, y):    try:        result = x / y        print("result is", result)    except ZeroDivisionError:        print("division by zero!")    finally:        print("executing finally clause")

divide(2, 1)# result is 2.0# executing finally clausedivide(2, 0)# division by zero!# executing finally clausedivide("2", "1")# executing finally clause# TypeError: unsupported operand type(s) for /: 'str' and 'str'

try:    检测范围except:    出现异常后的处理代码else:    如果没有异常执行这块代码

try:    fh = open("testfile.txt", "w")    fh.write("这是一个测试文件，用于测试异常!!")except IOError:    print("Error: 没有找到文件或读取文件失败")else:    print("内容写入文件成功")    fh.close()
# 内容写入文件成功

try:    raise NameError('HiThere')except NameError:    print('An exception flew by!')    # An exception flew by!