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 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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