Python入门系列（九）pip、try except、用户输入、字符串格式

pip

包含模块所需的所有文件。

检查是否安装了PIP

$ pip --version

安装包

$ pip install package_name

使用包

import package_name

删除包

$ pip uninstall camelcase

列出包

pip list

Try Except

try:
  print(x)
except:
  print("An exception occurred")

您可以根据需要定义任意数量的异常块，例如，如果您想为特殊类型的错误执行特殊代码块

try:
  print(x)
except NameError:
  print("Variable x is not defined")
except:
  print("Something else went wrong")

如果没有引发错误，可以使用else关键字定义要执行的代码块

try:
  print("Hello")
except:
  print("Something went wrong")
else:
  print("Nothing went wrong")

如果指定了finally块，则无论try块是否引发错误，都将执行finally。

ry:
  print(x)
except:
  print("Something went wrong")
finally:
  print("The 'try except' is finished")

这对于关闭对象和清理资源非常有用

try:
  f = open("demofile.txt")
  try:
    f.write("Lorum Ipsum")
  except:
    print("Something went wrong when writing to the file")
  finally:
    f.close()
except:
  print("Something went wrong when opening the file")

作为Python开发人员，如果出现条件，您可以选择抛出异常。

x = -1
if x < 0:
  raise Exception("Sorry, no numbers below zero")

您可以定义要引发的错误类型，以及要打印给用户的文本。

x = "hello"
if not type(x) is int:
  raise TypeError("Only integers are allowed")

用户输入

username = input("Enter username:")
print("Username is: " + username)

字符串格式

price = 49
txt = "The price is {} dollars"
print(txt.format(price)) # The price is 49 dollars

可以在花括号内添加参数，以指定如何转换值

price = 49
txt = "The price is {:.2f} dollars"
print(txt.format(price)) # The price is 49.00 dollars

如果要使用更多值，只需在format（）方法中添加更多值

quantity = 3
itemno = 567
price = 49
myorder = "I want {} pieces of item number {} for {:.2f} dollars."
print(myorder.format(quantity, itemno, price))

您可以使用索引号（大括号｛0｝内的数字）确保将值放置在正确的占位符中

quantity = 3
itemno = 567
price = 49
myorder = "I want {0} pieces of item number {1} for {2:.2f} dollars."
print(myorder.format(quantity, itemno, price))

此外，如果要多次引用同一值，请使用索引号

age = 36
name = "John"
txt = "His name is {1}. {1} is {0} years old."
print(txt.format(age, name))

命名索引

myorder = "I have a {carname}, it is a {model}."
print(myorder.format(carname = "Ford", model = "Mustang"))

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