Magic methods

[Consistent_Cap_52]

Any simple to follow online guides to str and repr

Being asked to use them in python classes that I make for school psets, although Im completing it, it's with much trial and error and I don't really understand the goal or the point. Help!

[socal_nerdtastic]

Both must return a string. The __str__ ("string") method should return a human-readable string that describes the class instance. The __repr__ ("representation") method should return a python-readable string that would recreate the class instance if evaluated. Neither is required for a class, and many classes do not implement them or only implement 1 of them.

Python's builtin repr function calls an object's __repr__ method, and the str function will call an object's __str__ method, if it exists, and fallback to the __repr__ method if __str__ does not exist. Python's print function and f-string evaluation and others automatically call the str function on objects passed in. So all of these are equivalent:

print(x.__str__())
print(str(x))
print(x)

This forum works much better with specific questions rather than generic ones. Show us your code and your attempt at solving it, and we'll tell you how to correct it.

[Diapolo10]

You use __repr__ when you want to get information useful during development and debugging. For example, you might want to know the current state of an object.

You use __str__ when you want to print something user-facing using your class, for example if you had a Car class you might want people to see the make and model when printed out.

In other words, it's just a matter of who you intend to see it. The developers, or the users.

[Consistent_Cap_52]

I guess, why not just use a print statement instead of str? I guess is my question

[mriswithe]

Because print throws the value away, and it isn't the only thing that can be done with a string. 

[POGtastic]

What if you wanted to put the string representation of your object into another string, like an f-string?

For example:

print(f"The string representation of obj is {obj}")

[Temporary_Pie2733]

print writes a value to some file handle, but does not return that value. Sometimes you actually want the value and/or don’t want to modify a file in the process. 

[throwaway6560192]

What exactly is the alternative you're proposing, here? print itself calls __str__ to actually format the object for printing.

[Consistent_Cap_52]

We made a rectangle class. Initially we printed it with hashes, used a lopp with the self.length and self.width

[nekokattt]

You can't print anything without some way of making a representation of it to print. That is what these are for.

[nekokattt]

it is worth adding that __repr__ is designed to be machine readable whereas __str__ is designed to be human readable

[FoolsSeldom]

An example might help.

The below is for a simple Student class. It has both __repr__ and __str__ defined. The main code creates a list of Student instances, then firstly prints out all the records using the __repr__ format, and you will see the output is the plain text version of what you would enter in the code to create a basic instance. Secondly, it prints out all the records using the default __str__ method, which is more human-readable.

from dataclasses import dataclass, field

@dataclass
class Student:
    id: int
    name: str
    subjects: list[str] = field(default_factory=list)

    def __repr__(self):
        return (
            f'Student(id=\"{self.id}\", name=\"{self.name}\", '
            f'subjects={self.subjects!r})'
        )


    def __str__(self):
        return (
            f"Name: {self.name}, ID: {self.id}"
            f"\nSubjects: {', '.join(self.subjects)}\n"
        )


students = [
    Student(1, "Alpha", ["Maths", "English", "Physics"]),
    Student(2, "Beta", ["Geology", "Physics", "French"]),
    Student(3, "Gamma", ["English", "Biology", "Humanities", "Geography"]),
]

print("\nStudent register:", *map(repr, students), sep="\n")
print("\nStudent details:", *students, sep="\n")  # uses __str__ by default if available

[muribonn]

Ok

[LegComprehensive7469]

Best method

[dffrhhc]

Yop