5. String Methods

We previously saw that each turtle instance has its own attributes and a number of methods that can be applied to the instance. For example, we wrote tess.right(90) when we wanted the turtle object tess to perform the right method to turn to the right 90 degrees. The dot notation is the way we connect the name of an object to the name of a method it can perform.

Strings are also objects. Each string instance has its own methods. There are a wide variety of str methods, some of which we’ve used already like ord and chr. Several more methods are shown in the table below. As an example, try the following program.

In this example, upper is a method that can be invoked on any string object to create a new string in which all the characters are in uppercase. The method lower works in a similar fashion by changing all characters in the string to lowercase. (The original string ss remains unchanged. A new string tt is created.)

In addition to upper and lower, the following table provides a summary of some other useful string methods. There are a few activecode examples that follow so that you can try them out.

Method Parameters Description
upper none Returns a string in all uppercase
lower none Returns a string in all lowercase
capitalize none Returns a string with first character capitalized, the rest lower
strip none Returns a string with the leading and trailing whitespace removed
lstrip none Returns a string with the leading whitespace removed
rstrip none Returns a string with the trailing whitespace removed
count item Returns the number of occurrences of item
replace old, new Replaces all occurrences of old substring with new
center width Returns a string centered in a field of width spaces
ljust width Returns a string left justified in a field of width spaces
rjust width Returns a string right justified in a field of width spaces
find item Returns the leftmost index where the substring item is found
rfind item Returns the rightmost index where the substring item is found
index item Like find except causes a runtime error if item is not found
rindex item Like rfind except causes a runtime error if item is not found

You should experiment with these methods so that you understand what they do. Note that the methods that return strings do not change the original. Strings are immutable (we will discuss this trait in more detail below). You can also consult the Python documentation for strings for additional details and more methods.

One method that you should experiment with is the str.format() method. This method will prove especially useful in Unit 2. This method can be called on a string in order to replace fields delimited by braces { }. Inside the braces is where you will put either the index of a positional argument, or the name of a keyword argument. Inside the format() call you put the variable or expression you want to add to the string. For example:

>>> "I have {0} pigs on my farm".format("eight")
'I have eight pigs on my farm'
>>> "I have {0} goats on my farm".format(1 + 8)
'I have 9 goats on my farm'

String Immutability

As we stated above, strings are immutable, which means you cannot change an existing string. The best you can do is create a new string that is a variation on the original.

It is tempting to use the [ ] operator on the left side of an assignment, with the intention of changing a character in a string. For example, in the following code, we would like to change the first letter of greeting.

Instead of producing the output Jello, world!, this code produces the runtime error TypeError: 'str' object does not support item assignment.

The solution here is to concatenate a new first letter onto a slice of greeting. This operation has no effect on the original string.

Check your understanding

    What is printed by the following statements?

    s = "python rocks"
print(s.count("o") + s.count("p"))
  • 0
  • There are definitely o and p characters.
  • 2
  • There are 2 o characters but what about p?
  • 3
  • Yes, add the number of o characters and the number of p characters.

    What is printed by the following statements?

    s = "python rocks"
print(s[1] * s.index("n"))
  • yyyyy
  • Yes, s[1] is y and the index of n is 5, so 5 y characters. It is important to realize that the index method has precedence over the repetition operator. Repetition is done last.
  • 55555
  • Close. 5 is not repeated, it is the number of times to repeat.
  • n
  • This expression uses the index of n
  • Error, you cannot combine all those things together.
  • This is fine, the repetition operator used the result of indexing and the index method.

    What is printed by the following statements:

    s = "Ball"
s[0] = "C"
print(s)
  • Ball
  • Assignment is not allowed with strings.
  • Call
  • Assignment is not allowed with strings.
  • Error
  • Yes, strings are immutable.