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    <p>First, you don't need to specify types in Python. So, if you want <code>method</code> to take a <code>First</code> instance, just do this:</p> <pre><code>class Second(): def __init__(self, c, d): pass def method(self, first): pass my_first = First(0, 1) my_second = Second(2, 3) my_second.method(my_first) </code></pre> <p>I believe that answers your real question, which is:</p> <blockquote> <p>If I have several instances of the class first and need to pass any of them not specifying which one to method of class Second…</p> </blockquote> <p>If you want to ensure that the parameter actually is a First, you can always add an <code>assert isinstance(first, First)</code> or <code>if not isinstance(first, First): raise TypeError</code> or whatever, but usually you don't want to do that in Python. The whole point of "duck typing" is that you write a function that takes "anything that acts like a <code>First</code> instance" rather than a function that takes "a <code>First</code> instance".</p> <hr> <p>You then say:</p> <blockquote> <p>Now I need to mutate variables from the First class inside the method of a second class:</p> </blockquote> <p>So… just do it. Your example doesn't have any attributes in the <code>First</code> class, so let's add some:</p> <pre><code>class First(): def __init__(self, a, b): self.total = a + b </code></pre> <p>And now, let's use them in <code>Second.method</code>:</p> <pre><code>class Second(): def __init__(self, c, d): self.total = c + d def method(self, first): first.total += self.total </code></pre> <p>So:</p> <pre><code>&gt;&gt;&gt; my_first = First(0, 1) &gt;&gt;&gt; my_first.total 1 &gt;&gt;&gt; my_second = Second(2, 3) &gt;&gt;&gt; my_second.total 5 &gt;&gt;&gt; my_first.total += 2 &gt;&gt;&gt; my_first.total 3 &gt;&gt;&gt; my_second.method(my_first) &gt;&gt;&gt; my_first.total 8 </code></pre> <hr> <p>Or, if you meant that you wanted to mutate the <em>class</em> attributes in class <code>First</code>… you don't even need a <code>First</code> instance for that:</p> <pre><code>First.my_class_attribute = 1 </code></pre> <hr> <p>If you really <em>do</em> need to pass a class itself… well, a class is a regular value like anything else:</p> <pre><code>class Second(): def __init__(self, c, d): pass def method(self, cls): pass my_second = Second(1, 2) my_second.method(First) </code></pre> <p>And you can access the class attributes of <code>cls</code> from within <code>method</code>, just as easily as you can access instance attributes when an instance is passed.</p>
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    1. POHow do I pass a whole class as a parameter to another class`s method
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    1. CONow I need to mutate variables from the First class inside the method of a second class:
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    2. COSo? Just do it! If you're doing the first version, where you have an object that should be a `First` instance, you can mutate its attributes through normal dot-syntax (or any other way you prefer). If you're doing the second, where you have an object that should be the `First` class itself, obviously you can only mutate the class attributes, both otherwise, it's the same thing.
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