python - string.Template与str.format()为什么同时存在

Question

这两段代码的效果是一样的:

from string import Template
template = Template('hi, ${name}')
msg = template.substitute(name=u'张三')
print msg

与

msg = u'hi, {name}'
msg = msg.format(name=u'张三')
print msg

我的问题是, string.Template与str.format谁的历史更久? 为什么会出现功能一样的库呢? 是不是一个是另一个的替代品呢?

Answer 1

The subject mentioned in my comment %，在str.format()There is this passage in the official document (str.format - python2):

This method of string formatting is the new standard in Python 3, and should be preferred to the % formatting described in String Formatting Operations in new code.

Probably means:

This method was added in version Python 3中是新标准，开发人员应该首先使用它，而不是%，来对字符串进行格式化操作。（format是2.6)

So the original question mentioned the alternative of Template和format是不是有替代关系，实际情况是，那俩家伙并没有，反而format却真的是%.

Since the official said so, I recommend that you use it format就不要用% when it comes to string formatting operations in the future.

Original answer:

Template是string模块里的类，format是__buildin__Built-in functions in the module, this is the fundamental difference between the two.

Since it is a class, you can inherit it and rewrite the content according to your own needs. For example, the default delimiter $ can be modified by us:

from string import Template

class MyTemp(Template):
    delimiter = '#'

tem = MyTemp('hi, #{name}')
msg = tem.substitute(name='Jake')

print msg

Output result:

hi, Jake

In the same way, we can also do more things we want. This is the "private customization" that can be achieved as a class.

In addition, regarding format, its application range is actually very wide. In addition to what you mentioned in your example, we also commonly use the following:

print 'Hi, {0}, {1}.'.format('Jake', 'Tom')
ppl = ['Jake', 'Tom']
print 'Hi, {0[0]}, {0[1]}.'.format(ppl)
# 输出结果都是 Hi, Jake, Tom.

And the very important padding alignment, precision, and even base conversion:

print '{:>5}'.format(100)
print '{:x>10}'.format(100)
print '{:.2f}'.format(100.02111)
print '{:b}'.format(15)

# 输出结果：
  100
xxxxxxx100
100.02
1111

So the application directions of Template and Template和format are completely different.

Answer 2

Why aren’t you surprised why the sorted function and list.sort() exist at the same time?
Functions with similar functions are generally aimed at special application scenarios, for example, sorted has a return value and sort directly changes the object.

a = [1,3,2]
b = sorted(a)
b == [1,2,3]
a == [1,3,2]
a.sort()
a == [1,2,3]

For string.Template and str.format, string.Template, as its name states, is suitable for defining templates and can be used later. For example, if you define a template in a function or package, it can be conveniently used at any time. Called without knowing its details. The str.format() is generally used for single-line expressions and is more flexible to use.