WORKING WITH STRINGS

You can concatenate two strings using the “+” operator. The following example prints a string and then concatenates two single-letter strings:

>>> 'abc'
'abc'
>>> 'a' + 'b'
'ab'

You can use “+” or “*” to concatenate identical strings, as shown here:

>>> 'a' + 'a' + 'a'
'aaa'
>>> 'a' * 3
'aaa'

You can assign strings to variables and print them using the print() command:

>>> print('abc')
abc
>>> x = 'abc'
>>> print(x)
abc
>>> y = 'def'
>>> print(x + y)
abcdef

You can “unpack” the letters of a string and assign them to variables, as shown here:

>>> str = "World"
>>> x1,x2,x3,x4,x5 = str
>>> x1
'W'
>>> x2
'o'
>>> x3
'r'
>>> x4
'l'
>>> x5
'd'

The preceding code snippets show you how easy it is to extract the letters in a text string, and in Chapter 3 you will learn how to “unpack” other data structures.

You can extract substrings of a string as shown in the following examples:

>>> x = "abcdef"
>>> x[0]
'a'
>>> x[-1]
'f'
>>> x[1:3]
'bc'
>>> x[0:2] + x[5:]
'abf'

However, you will cause an error if you attempt to “subtract” two strings:

>>> 'a' - 'b'
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: unsupported operand type(s) for -: 'str' and 'str'

The try/except construct enables you to handle the preceding type of exception gracefully.

Comparing Strings

You can use the methods lower() and upper() to convert a string to lowercase and uppercase, respectively, as shown here:

>>> 'Python'.lower()
'python'
>>> 'Python'.upper()
'PYTHON'
>>>

The methods lower() and upper() are useful for performing a case-insensitive comparison of two ASCII strings. Listing 1.2 displays the content of compare.py that uses the lower() function to compare two ASCII strings.

LISTING 1.2: compare.py

x = 'Abc'
y = 'abc'

if(x == y):
  print('x and y: identical')
elif (x.lower() == y.lower()):
  print('x and y: case insensitive match')
else:
  print('x and y: different')

Since x contains mixed case letters and y contains lowercase letters, Listing 1.2 displays the following output:

x and y: different

Formatting Strings

Python provides the functions string.lstring(), string.rstring(), and string.center() for positioning a text string so that it is left-justified, right-justified, and centered, respectively. As you saw in a previous section, Python also provides the format() method for advanced interpolation features.

Now enter the following commands in the interpreter:

import string

str1 = 'this is a string'
print(string.ljust(str1, 10))
print(string.rjust(str1, 40))
print(string.center(str1,40))

The output is shown here:

this is a string
                        this is a string
            this is a string

The next portion of this chapter shows you how to “slice and dice” text strings with built-in functions.