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Mosh的Python课程笔记(6)--Exceptions

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Mosh的课程网址

Handling Exceptions

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try:
    age = int(input("Age: "))
except ValueError as ex:
    print("You didn't enter a valid age.")
    print(ex)			# 输出错误信息
    print(type(ex))		# 显示错误类型
else:					# 没有exception就执行这一块
    print("No exceptions were thrown.")

print("Execution continues.")

运行结果：

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>python app.py
Age: 10
No exceptions were thrown.
Execution continues.

>python app.py
Age: a
You didn't enter a valid age.
invalid literal for int() with base 10: 'a'   
<class 'ValueError'>
Execution continues.

Handling Different Exceptions

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try:
    age = int(input("Age: "))
    xfactor = 10 / age
except (ValueError, ZeroDivisionError):			# 多种exception之一即执行
    print("You didn't enter a valid age.")
else:
    print("No exceptions were thrown.")

输出结果：

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> python app.py
Age: 0
You didn't enter a valid age.

Cleaning Up

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try:
    file = open("app.py")
    age = int(input("Age: "))
    xfactor = 10 / age
except (ValueError, ZeroDivisionError):
    print("You didn't enter a valid age.")
else:
    print("No exceptions were thrown.")
finally:
    file.close()

无论有没有出现exception，都要释放资源，那就放在finally块里，无论有没有exception都会执行的。

The With Statement

使用with语句打开文件，随后会自动释放掉，就不用file.close()了：

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with open("app.py") as file:
       print("File opened.")

file有__enter__和__exit__两个magic function，最后会自动调用__exit__，所以不用手动释放。

Raising Exceptions

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def calculate_xfactor(age):
    if age <= 0:
        raise ValueError("Age cannot be 0 or less.")
    return 10 / age


try:
    calculate_xfactor(-1)
except ValueError as error:
    print(error)

Cost of Raising Exceptions

Mosh 不建议raise exceptions，因为代价有点大。

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from timeit import timeit

code1 = """
def calculate_xfactor(age):
    if age <= 0:
        raise ValueError("Age cannot be 0 or less.")
    return 10 / age


try:
    calculate_xfactor(-1)
except ValueError as error:
    pass
"""

code2 = """
def calculate_xfactor(age):
    if age <= 0:
        return None
    return 10 / age


xfactor = calculate_xfactor(-1)
if xfactor == None:
    pass
"""

print("first code=", timeit(code1, number=10000))
print("second code=", timeit(code2, number=10000))

输出结果：

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first code= 0.0040202000000000016
second code= 0.0015166999999999958

可以看到raise exception的用时是普通方法的两倍多。