Section 7.9 String methods
Strings are an example of Python
objects. An object contains both data (the actual string itself) and
methods, which are effectively functions that are built into the object and are available to any
instance of the object.
Python has a function called
dir which lists all the methods available for an object. The
type function returns the type of an object.
>>> help(str.capitalize)
Help on method_descriptor:
capitalize(...)
S.capitalize() -> str
Return a capitalized version of S, i.e. make the first character
have upper case and the rest lower case.
>>>
Checkpoint 7.9.1.
Calling a
method is similar to calling a function (it takes arguments and returns a value), but the syntax is different. We call a method by typing the variable name, adding a period, and then adding the method call.
For example, the method
upper takes a string and returns a new string with all uppercase letters. Instead of the function syntax,
upper(word), it uses the method syntax,
word.upper().
This form of dot notation specifies the name of the method,
upper, and the name of the string to apply the method to,
word. The empty parentheses indicate that this method takes no argument.
A method call is called an
invocation; in this case, we would say that we are invoking
upper on the
word.
For example, there is a string method named
find that searches for the position of one string within another:
In this example, we invoke
find on
word and pass the letter we are looking for as a parameter.
The
find method can find substrings as well as characters:
It can also take as a second argument the index where it should start looking:
One common task is to remove white space (spaces, tabs, or newlines) from the beginning and end of a string using the
strip method:
Some methods, such as
startswith, return boolean values:
You will note that
startswith requires case to match, so sometimes we'll convert a line to lowercase using the
lower method before we do any checking:
In the last example, the method
lower is called. Then, we use
startswith to see if the resulting lowercase string starts with the letter “h”. As long as we are careful with the order, we can make multiple method calls in a single expression.
Checkpoint 7.9.2.
Checkpoint 7.9.3.
Checkpoint 7.9.4.
Match these common string methods with their function. Hint: many of the names of these methods are hints to what they do!
If you need help, try looking online for the effects of these methods.
- capitalize()
- Returns a copy of a string with its first character capitalized and the rest lowercased.
- count()
- Returns the number of non-overlapping occurrences of a substring in the range [start, end].
- endswith()
- Returns True if a string ends with the specified suffix. Otherwise, it returns False.
- find()
- Returns the lowest index in a string where a substring is found. An optional parameter restricts the search to a slice of the string.
- strip()
- Returns a copy of a string with the leading and trailing whitespace characters removed.
- upper()
- Returns a copy of a string with all characters converted to uppercase.
https://docs.python.org/library/stdtypes.html#string-methods
