Strings and String Methods
Strings are objects belonging to the str class and possess methods for checking, searching, and transforming text.
Text with Built-In Behavior
A string is more than a sequence of visible characters. In Python, a string is an object belonging to the str class. Because it belongs to that class, it possesses methods for checking, searching, and transforming text. This lets you ask questions about text and create new text using operations attached to the string object itself.
The three methods in this lesson provide three different actions: startswith() checks a prefix, find() searches for a substring, and join() combines strings using a delimiter.
A String and Its Methods
When you work with a string, its methods are operations provided by the str class. The method call begins with a string value, followed by a dot and the method name. For example, a string can be used to call startswith(), find(), or join(). These methods return values. They do not modify the original string because strings are immutable.
Checking Prefixes with startswith()
The startswith() method tests whether a string begins with specified characters. Its result is a Boolean value: True when the beginning matches the prefix and False when it does not. This makes the method useful when a task depends on how text begins.
True
FalseIn this example, the first call asks whether filename begins with report, so it produces True. The second call checks for data at the beginning, so it produces False. The method checks the prefix rather than searching for the text anywhere in the string.
Finding a Substring Position
The find() method searches for a substring inside a string and returns the character position where that substring begins. The returned position refers to the original string, so you can use it to understand where the match starts.
Locating a Word
Find the starting position of the substring "thon" in the string "Python".
Search: Use find() on the string "Python" with "thon" as the substring being searched for.
Map the match: The substring begins after the first two characters, so its starting position is 2.
Read the result: The method returns the starting character position rather than the matched text itself.
The starting position is 2.
2Combining Text with join()
The join() method combines a sequence of strings into one string. The string on which you call join() becomes the delimiter placed between the sequence elements. This means the method is organized around the separator: choose the separator first, then use it to join the strings.
Ada, LovelaceHere, ", " is the delimiter and parts is the sequence being combined. The result is a new string containing the sequence elements with the delimiter between them.
Exploring the str Class
The three methods in this lesson are only part of the behavior available on str objects. Use the built-in help() function with str to explore the full range of string methods and read their documentation.
When you run help(str), Python exposes documentation for the str class, including the methods available for working with strings. Treat it as a reference to consult when you need a string operation that is not covered by the methods you already know.
Mistakes with String Methods
Expecting startswith() to search anywhere in the string.
startswith() tests the beginning of the string, not an arbitrary location within it.
Fix:
Use find() when the task is to locate a substring and obtain its starting position.Expecting find() to return the matched substring.
find() locates the substring and returns its character position.
Fix:
Interpret the result as the starting position of the match.Expecting a string method to change the original string.
String methods return new values, and strings are immutable.
Fix:
Store or use the returned value when you need to keep the result.Choosing the wrong object as the join separator.
The string on which join() is called becomes the delimiter between the sequence elements.
Fix:
Call join() on the separator, such as ", ".join(parts).
Practice: Choose the Method
For each task, choose startswith(), find(), or join(). Then describe what kind of result the method should produce: a Boolean, a character position, or a new combined string.
Hints
- A prefix check asks about the beginning of a string.
- A search for a substring asks where text begins.
- Combining several strings requires a delimiter and a sequence.
Applying the Three Methods
Choose the appropriate method for these tasks: check whether "invoice_7" begins with "invoice", locate "7" in "invoice_7", and combine "invoice" and "7" with an underscore.
Check the beginning: Use startswith() because the task asks whether the string begins with a particular prefix.
Locate text: Use find() because the task asks for the position of a substring.
Combine parts: Use join() with an underscore as the delimiter because the task asks for one string made from multiple strings.
The methods are startswith(), find(), and join(), respectively. Their results are a Boolean, a position, and a newly combined string.
What to Remember
- Strings are objects belonging to the str class.
- startswith() checks whether a string begins with a specified prefix and returns a Boolean result.
- find() searches for a substring and returns its starting character position.
- join() combines a sequence of strings using the string on which join() is called as the delimiter.
- String methods return new values rather than modifying the original string, and help(str) provides documentation for discovering more methods.
Key Takeaways
- A Python string is an object belonging to the str class.
- Use startswith() for prefix checks, find() for substring positions, and join() for combining strings.
- String methods return new values because strings are immutable.
- Use help(str) to discover and read documentation for additional string methods.