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New example: The chicken-egg problem

Closes https://github.com/satwikkansal/wtfpython/issues/101
This commit is contained in:
Satwik Kansal 2019-06-09 00:07:02 +05:30
parent 72e39d3248
commit 9ea7ab0c5a

48
README.md vendored
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@ -730,6 +730,54 @@ Even when the values of `x` were different in every iteration prior to appending
--- ---
### ▶ The chicken-egg problem ^
1\.
```py
>>> isinstance(3, int)
True
>>> isinstance(type, object)
True
>>> isinstance(object, type)
True
```
2\. So which is the ultimate, base class? And wait, there's more to the confusion
```py
>>> class A: pass
>>> isinstance(A, A)
False
>>> isinstance(type, type)
True
>>> isinstance(object, object)
True
```
3\.
```py
>>> issubclass(int, object)
True
>>> issubclass(type, object)
True
>>> issubclass(object, type)
False
```
#### 💡 Explanation
- `type` is a [metaclass](https://realpython.com/python-metaclasses/) in Python.
- **Everything** is an `object` in Python, which includes classes as well as their objects (instances).
- class `type` is the metaclass of class `object`, and every class (including `type`) has inherited directly or indirectly from `object`.
- There is no real base class among `object` and `type`. The confusion in the above snippets is arising because we're thinking these relationships (`issubclass` and `isinstance`) in terms of Python classes. The relationship between `object` and `type` can't be reproduced in pure python. To be more precise the following relationships can't be reproduced in pure Python,
+ class A is instance of class B, and class B is an instance of class A.
+ class A is an instance of itself.
- These relationships between `object` and `type` (both being instances of eachother as well as themselves) exist in Python because of "cheating" at implementation level.
---
### ▶ `is not ...` is not `is (not ...)` ### ▶ `is not ...` is not `is (not ...)`
```py ```py