Basic Operations

Let’s begin by interacting with the Python interpreter to illustrate the basic string operations listed earlier in Table 7-1. Strings can be concatenated using the + operator and repeated using the * operator:

% python
>>> len('abc')            # Length: number of items
3
>>> 'abc' + 'def'         # Concatenation: a new string
'abcdef'
>>> 'Ni!' * 4             # Repetition: like "Ni!" + "Ni!" + ...
'Ni!Ni!Ni!Ni!'

广告：个人专属 VPN，独立 IP，无限流量，多机房切换，还可以屏蔽广告和恶意软件，每月最低仅 5 美元

Formally, adding two string objects creates a new string object, with the contents of its operands joined. Repetition is like adding a string to itself a number of times. In both cases, Python lets you create arbitrarily sized strings; there’s no need to predeclare anything in Python, including the sizes of data structures.[19] The len built-in function returns the length of a string (or any other object with a length).

Repetition may seem a bit obscure at first, but it comes in handy in a surprising number of contexts. For example, to print a line of 80 dashes, you can count up to 80, or let Python count for you:

>>> print('------- ...more... ---')      # 80 dashes, the hard way
>>> print('-' * 80)                      # 80 dashes, the easy way

Notice that operator overloading is at work here already: we’re using the same + and * operators that perform addition and multiplication when using numbers. Python does the correct operation because it knows the types of the objects being added and multiplied. But be careful: the rules aren’t quite as liberal as you might expect. For instance, Python doesn’t allow you to mix numbers and strings in + expressions: 'abc'+9 raises an error instead of automatically converting 9 to a string.

As shown in the last row in Table 7-1, you can also iterate over strings in loops using for statements and test membership for both characters and substrings with the in expression operator, which is essentially a search. For substrings, in is much like the str.find() method covered later in this chapter, but it returns a Boolean result instead of the substring’s position:

>>> myjob = "hacker"
>>> for c in myjob: print(c, end=' ')   # Step through items
...
h a c k e r
>>> "k" in myjob                        # Found
True
>>> "z" in myjob                        # Not found
False
>>> 'spam' in 'abcspamdef'              # Substring search, no position returned
True

The for loop assigns a variable to successive items in a sequence (here, a string) and executes one or more statements for each item. In effect, the variable c becomes a cursor stepping across the string here. We will discuss iteration tools like these and others listed in Table 7-1 in more detail later in this book (especially in Chapters 14 and 20).