9.5. 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 attributes and methods. The most important attribute of the string is the collection of characters. There are a wide variety of methods. Try the following program.

ss = "Hello, World" print(ss.upper()) tt = ss.lower() print(tt)

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. lower works in a similar fashion 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 once again that the methods that return strings do not change the original. You can also consult the Python documentation for strings.

ss = " Hello, World " els = ss.count("l") print(els) print("***" + ss.strip() + "***") print("***" + ss.lstrip() + "***") print("***" + ss.rstrip() + "***") news = ss.replace("o", "***") print(news) food = "banana bread" print(food.capitalize()) print("*" + food.center(25) + "*") print("*" + food.ljust(25) + "*") # stars added to show bounds print("*" + food.rjust(25) + "*") print(food.find("e")) print(food.find("na")) print(food.find("b")) print(food.rfind("e")) print(food.rfind("na")) print(food.rfind("b")) print(food.index("e"))

Check your understanding

strings-5-1: What is printed by the following statements?

s = "python rocks"
print(s.count("o") + s.count("p"))

• (A) 0
• There are definitely o and p characters.
• (B) 2
• There are 2 o characters but what about p?
• (C) 3
• Yes, add the number of o characters and the number of p characters.

strings-5-2: What is printed by the following statements?

s = "python rocks"
print(s[1] * s.index("n"))

• (A) 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.
• (B) 55555
• Close. 5 is not repeated, it is the number of times to repeat.
• (C) n
• This expression uses the index of n
• (D) Error, you cannot combine all those things together.
• This is fine, the repetition operator used the result of indexing and the index method.