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Python中实现替换字符串中的子串

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