在Python中实现替换字符串中的子串的示例

假如有个任务： 给定一个字符串，通过查询字典，来替换给定字符中的变量。如果使用通常的方法：

>>> "This is a %(var)s" % {"var":"dog"}'This is a dog'>>>

其实可以使用string.Template类来实现上面的替换

>>> from string import Template>>> words = Template("This is $var")>>> print(words.substitute({"var": "dog"})) # 通过字典的方式来传参This is dog>>> print(words.substitute(var="dog"))   # 通过关键字方式来传参This is dog>>>

在创建Template实例时，在字符串格式中，可以使用两个美元符来代替$，还可以用${}将 变量扩起来，这样的话，变量后面还可以接其他字符或数字，这个使用方式很像Shell或者Perl里面的语言。下面以letter模板来示例一下:

>>> from string import Template>>> letter = """Dear $customer,... I hope you are having a great time!... If you do not find Room $room to your satisfaction, let us know.... Please accept this $$5 coupon....     Sincerely,...     $manager,...     ${name}Inn""">>> template = Template(letter)>>> letter_dict = {"name": "Sleepy", "customer": "Fred Smith", "manager": "Tom Smith", "room": 308}>>> print(template.substitute(letter_dict))Dear Fred Smith,I hope you are having a great time!If you do not find Room 308 to your satisfaction, let us know.Please accept this $5 coupon.    Sincerely,    Tom Smith,    SleepyInn>>>

有时候，为了给substitute准备一个字典做参数，最简单的方法是设定一些本地变量，然后将这些变量交给local()(此函数创建一个字典，字典中的key就是本地变量，本地变量的值通过key来访问)。

>>> locals()   # 刚进入时，没有其他变量{'__builtins__': <module '__builtin__' (built-in)>, '__name__': '__main__', '__doc__': None, '__package__': None}>>> name = "Alice" # 创建本地变量name >>> age = 18   # 创建本地变量age>>> locals()   # 再执行locals()函数就可以看到name, age的键值队{'name': 'Alice', '__builtins__': <module '__builtin__' (built-in)>, 'age': 18, '__package__': None, '__name__': '__mai__', '__doc__': None}>>> locals()["name"] # 通过键name来获取值'Alice'>>> locals()["age"] # 通过键age来获取值18>>>

有了上面的例子打底来看一个示例：

>>> from string import Template>>> msg = Template("The square of $number is $square")>>> for number in range(10):...  square = number * number...  print msg.substitute(locals())...The square of 0 is 0The square of 1 is 1The square of 2 is 4The square of 3 is 9

另外一种方法是使用关键字参数语法而非字典，直接将值传递给substitute。

>>> from string import Template>>> msg = Template("The square of $number is $square")>>> for i in range(4):...  print msg.substitute(number=i, square=i*i)...The square of 0 is 0The square of 1 is 1The square of 2 is 4The square of 3 is 9>>>

甚至可以同时传递字典和关键字

>>> from string import Template>>> msg = Template("The square of $number is $square")>>> for number in range(4):...  print msg.substitute(locals(), square=number*number)...The square of 0 is 0The square of 1 is 1The square of 2 is 4The square of 3 is 9>>>

为了防止字典的条目和关键字参数显示传递的值发生冲突，关键字参数优先，比如:

>>> from string import Template>>> msg = Template("It is $adj $msg")>>> adj = "interesting">>> print(msg.substitute(locals(), msg="message"))It is interesting message 

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