Debugging state
Inspecting objects in a debugger or interactive session.
Python · Module
__repr__
__repr__ defines an object's official string representation for repr(). Make it information-rich and unambiguous, ideally resembling Python code that can recreate the value.
How it works
Define __repr__(self) to return the string produced when repr() represents an instance. Prefer a clear, information-rich result that resembles a Python expression able to recreate the value; if that is not feasible, return a useful angle-bracket description. When __str__ is absent, Python also uses this method for informal string conversion.
Syntax
object.__repr__(self)Parameters
| Name | Type | Description |
|---|---|---|
self | object | The instance whose official representation is being computed. |
Returns
A string object containing the object's official representation.
Key points
- Python invokes __repr__ through repr() to obtain an object's official textual representation.
- A strong representation resembles executable Python that could reconstruct an equivalent object in the right environment.
- When reconstruction syntax is unavailable, return a useful angle-bracket description instead.
- Optimize the output for debugging by including enough detail to identify the object's state without ambiguity.
- Classes that do not customize the method inherit a default implementation from object.
Common mistakes
- Returning a non-string value from __repr__ is invalid and raises an error when the representation is requested.
- Defining only __repr__ also controls the class's informal string representation, so it may appear in contexts where a concise display was expected.
Example: A useful object representation
class User:
def __init__(self, name, role):
self.name = name
self.role = role
def __repr__(self):
return f"User(name={self.name!r}, role={self.role!r})"
user = User("Ada", "admin")
print(repr(user))The method returns a readable representation containing constructor-like field values.
When to use it
Diagnostic logging
Logging domain objects with identifying fields.
Reconstructible display
Producing constructor-like output for value-oriented classes.
Interview questions
QWhat is __repr__ used for?
__repr__ is called by repr() to compute an object's official string representation. It should be information-rich and unambiguous, ideally resembling a valid Python expression that can recreate the value.
Follow-up: What should it return if recreation syntax is impractical? Return a string shaped like <...some useful description...>.
QWhat must __repr__ return?
The method's result must be a string. A class can implement it as def __repr__(self), and the built-in repr() invokes it for instances.
Follow-up: What happens if the method returns an integer? It violates the required return type and representation fails.
QWhat is the relationship between __repr__ and __str__?
If __str__ is not defined, Python uses __repr__ when an informal string representation is needed. This makes a sole __repr__ implementation affect both debugging output and informal display.
Follow-up: How does this differ from __str__? __str__ has no expectation of returning a valid Python expression and may be more convenient or concise.
Related
repr()object.__str__()str()object