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

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 0
The square of 1 is 1
The square of 2 is 4
The 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 0
The square of 1 is 1
The square of 2 is 4
The 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 0
The square of 1 is 1
The square of 2 is 4
The 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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