Debugging with Built-in Functions
The built-in functions type() and dir() provide two useful ways to investigate: type() identifies the object's class or type, while dir() lists the methods and attributes available on it.
When a Value Is Unfamiliar
When debugging or learning Python, you may encounter a value without knowing exactly what kind of object it is or what you can do with it. The built-in functions type() and dir() provide two useful ways to investigate: type() identifies the object's class or type, while dir() lists the methods and attributes available on it.
Start by identifying what the object is. Then inspect the operations and attributes available for that kind of object.
Identifying the Object
The type() function returns the class or type of an object. This tells you what kind of data you are working with. For instance, when investigating an unfamiliar value, type() helps distinguish whether you are working with a string, a list, a number, or another kind of object.
Finding the Nature of an Unfamiliar Value
You receive an unfamiliar Python object while debugging and need to determine what kind of data it represents.
- 1Inspect the value with type()
Use
type()on the object. The result identifies the object's class or type. - 2Interpret the result
Use the identified type to form a first expectation about the data and the operations that may be available.
- 3Continue with dir()
After identifying the type, inspect the object's available methods and attributes with
dir().
type() answers the question 'What kind of object is this?' before you investigate what that object can do.
Discovering Available Capabilities
The dir() function lists the methods and attributes available for an object. This makes it useful when you know the object exists but do not know which operations or pieces of information it provides. The result is an inventory for exploration: it shows what is available on that object.
Suppose you are working with an unfamiliar list-like or string-like value and cannot remember which operations it supports. type() first helps identify the kind of object. dir() then gives you a list of the methods and attributes associated with that object, helping you decide what to investigate next.
Why Type Controls Operations
An object's type directly determines which methods are available to it. Different types have different method sets, so an operation that is available for one kind of object may not appear for another. This is why identifying the type is an important first step when an operation is unfamiliar or does not behave as expected.
| Investigation question | Built-in function | What it reveals |
|---|---|---|
| What kind of object is this? | type() | The object's class or type |
| What can I access or do with this object? | dir() | Methods and attributes available on the object |
| What does this method mean or require? | help() | Quick documentation for the method |
Each function answers a different debugging question.
A Practical Investigation Sequence
- 1Use
type()to determine what kind of object you are examining. - 2Use
dir()to discover the methods and attributes available for that object. - 3Select a method or attribute that appears relevant to the problem.
- 4Use
help()for quick documentation about the method. - 5Consult the official Python documentation when you need more comprehensive details.
Investigating an Unfamiliar Object
You want to perform an operation on an unfamiliar object but do not know whether the object supports it.
- 1Identify
Apply
type()to learn the object's class or type. - 2Explore
Apply
dir()to see the methods and attributes available for that object. - 3Document
Use
help()on a relevant discovered method to obtain quick documentation. - 4Verify
Use the official Python documentation when the quick explanation is not enough and comprehensive details are needed.
The investigation moves from the object's identity to its available capabilities and then to reliable usage information.
Documentation After Discovery
After dir() helps you discover a relevant method, help() provides quick documentation for that method. When you need comprehensive details, consult the official Python documentation. This creates a useful progression: dir() helps you find a possibility, help() helps you understand it quickly, and official documentation provides a deeper reference.
| Tool or resource | Best question it answers |
|---|---|
| type() | What class or type is this object? |
| dir() | Which methods and attributes are available? |
| help() | What quick documentation is available for this method? |
| Official Python documentation | Where can I find comprehensive details? |
Use the tool that matches the question you are asking.
Mistakes During Investigation
Common mistake Trying to choose a method before identifying the object's type
What it looks likeYou look for an operation without first checking what kind of object you have.
Why it failsDifferent types have different method sets, so the available operations depend on the object's type.
Do this insteadUse type() first, then use dir() to inspect the methods and attributes for that object.
Common mistake Treating dir() as complete documentation
What it looks likeYou find a method name in the dir() result and assume you already know how to use it.
Why it failsdir()lists available methods and attributes, but the source identifieshelp()and official Python documentation as the places to learn about a method.Do this insteadUse help() for quick documentation and consult the official Python documentation for comprehensive details.
Common mistake Assuming every object has the same methods
What it looks likeYou expect a method available for one type to be available for every other type.
Why it failsAn object's type directly determines which methods are available, and different types have different method sets.
Do this insteadInspect each unfamiliar object's type and available members instead of relying on assumptions.
Practice: Investigate Before Acting
Imagine that a debugging session gives you an unfamiliar Python object. Describe the order in which you would use type(), dir(), and help(). For each step, state the question that function answers.
Hints
- Begin with the function that identifies the object's class or type.
- Next, identify the function that lists methods and attributes.
- Finish with the function or resource used to understand a discovered method.
What do you think happens?
You have identified an unfamiliar object's type and found a potentially useful method with dir(). What should you use next when you need a quick explanation of that method?
Reveal answer
help()
help() provides quick documentation for a method. The official Python documentation can provide more comprehensive details.
Key Takeaways
- 1
type()returns the class or type of an object. - 2
dir()lists the methods and attributes available on an object. - 3An object's type determines which methods are available to it.
- 4Use
help()for quick method documentation. - 5Use the official Python documentation when you need comprehensive details.
At a glance
- Use
type()to identify what kind of Python object you are investigating. - Use
dir()to discover the methods and attributes available for that object. - Remember that different object types have different method sets.
- Use
help()and official Python documentation to understand discovered methods. - A reliable debugging sequence is identify, discover, document, and verify.