Data Classes in python 😈(part-2)

In this Blog, Let’s discuss dataclass decorator parameters. We can control the features of the dataclass by passing arguments to dataclass decorator .

Prerequisite : Please Read Part-1(Hyper Link)

In this Blog, Let’s discuss dataclass decorator parameters. We can control the features of the dataclass by passing arguments to dataclass decorator . The syntax of the dataclass decorator is :

@dataclasses.dataclass(*,init=True, repr=True, eq=True, order=False,unsafe_hash=False, frozen=False,match_args=True, kw_only=True, slots=False)

What is this bro ? Looks so complex ! 😕. I’ll make you guys understand easily. Out of these 9 parameters , 6 are important . Let’s focus 3 in this blog and rest of the 3 in the upcoming blog (Data Classes in python (part-3)).


Let’s discuss internal __init__ or default constructor

init = True, means Dataclass uses internal constructor to initialize the defined data by the user . This is by default in data classes. If i make init = False, I can no longer initialize values for my object.

Yes, It raises TypeError. So, I handled using Exceptions Handling.

output of the above code :

As, We did init = False in dataclass decorator, python raises error that Dataclass won’t take any arguments.

Now, __repr__ won’t work . Why ?

__repr__ relies on internal __init__ . As long as internal __init__ won’t take any data for initializing , __repr__ will not allowed to print the objects associated data.

You may get a doubt as , What if i create my own constructor when data class is in init = False mode ? Do the internal __repr__ works now ?

Yes internal __repr__ works when i create my own __init__ but , the data will not be accepted by internal __repr__ as internal __repr__ is only bounded to the internal __init__ of a data class. Look below to understand more deeply…

Can you guess what the last line prints ?

It prints “FavFood()” because internal __init__ is in ‘False’ mode. So __repr__will not consider external __init__ we created .

output of the above code :


Let’s discuss what happens when internal __repr__ in False mode

In the above code __repr__ in ‘False’ mode . So when i print the object , i won’t return the user understandable string with the objects data in it .

Let’s see what i will get !

The output with __repr__ in ‘False’ mode is not understandable right ? 😅So, Never use __repr__ in ‘False’ mode.


Let’s discuss the last topic in this blog . i.e, “eq” parameter

In the first blog on data classes i said that “2 or more data class objects with same data will always evaluate to true on == operator”. If i make eq = False ,then this property will no longer applicable to data class objects.

Generally we will get ‘True’ for the above code output. As, eq = False , data class decorator will no longer compare 2 objects by taking their associated data in them. But, the 2 objects are compared based on their hash value.


The remaining 3 important parameters will be discussed in the coming blog (Data Classes in python (part-3)).

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Will post Data classes in python (part-3) soon……..


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yokesh Chowdary

Backend developer, Clarivate

@yokee-ss-hh
Loves Backend web development, Consistently playing with Datastructures and algorithms.
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