Python函数参数和文档混乱

Question

Here is a string in python:

a = "asdf as df adsf as df asdf asd f"

Lets say that I want to replace all " " with "||", so I do:

>>> a.replace(" ", "||")
'asdf||as||df||adsf||as||df||asdf||asd||f'

My confusion is info from the documentation as below:

 string.replace(s, old, new[, maxreplace])
    Return a copy of string s with all occurrences...

I can "omit" s , but based on the documentation I need s ; however, I only provide old , and new . I noticed that it's like this with a lot of the python documentation; what am I missing?

Answer 1

You are mixing up str object methods with the string module functions.

The documentation you are referring to is the string module documentation. Indeed, there is a function in the string module called replace which takes 3 (or optionally, 4) arguments:

In [9]: string
Out[9]: <module 'string' from '/usr/lib/python2.7/string.pyc'>

In [11]: string.replace(a, ' ', '||')
Out[11]: 'asdf||as||df||adsf||as||df||asdf||asd||f'

a is a str object -- ( str is a type, string is a module):

In [15]: type(a)
Out[15]: str

And str objects have a replace method. The documentation for the str methods is here .

Answer 2

When you call a method of an object, the object is supplied automatically as the first parameter. Generally within the method this is referred to as self .

So you can call the function passing in the object:

string.replace(s, old, new)

or you can call the method of the object:

s.replace(old, new)

Both are functionally identical.

Answer 3

The first parameter of a method is a reference to the object (usually called self ) being modified and is implicitly passed when you use the object.method(...) notation. So this:

a = "asdf as df adsf as df asdf asd f"
print a.replace(" ", "||")

is equivalent to this:

a = "asdf as df adsf as df asdf asd f"
print str.replace(a, " ", "||")

str being the class of the a object. It's just syntactical sugar.