Pandas: What is a view?

Question

Please help me to understand: what is a view in Pandas. I know that if we change something in a view we always make changes in the original object.

But a view of an object and the original object have different id's for example. Does it mean that the view is another object with reference to original object? What is the mechanism?

I tried but can't find an explanation.

import pandas as pd
import numpy as np

df = pd.DataFrame({'x': [1,2]})
print(df)
df_sub = df[0:1]
df_sub.x = -1
print(df_sub._is_view)               # True
print(id(df) == id(df_sub))          # False
print(np.shares_memory(df, df_sub))  # True

Answer 1

To understand what a View is, you have to know what an arrays is. An array is not only the "stuff" (items) you put in it. It needs (besides others) also information about the number of elements, the shape of your array and how to interpret the elements.

So an array would be an object at least containing these attributes:

class Series:
    data    # A pointer to where your array is stored
    size    # The number of items in your array
    shape   # The shape of your array
    dtype   # How to interpret the array

So when you create a view a new array object is created but (and that's important) the View's data pointer points to the original array. It could be offset but it still points to one memory location that belongs to the original array. But even though it shares some data with the original the size, shape, dtype (, ...) might have changed so it requires a new object. That's why they have different ids.

Think of it like windows. You have a garden (the array) and you have several windows, each window is a different object but all of them look out at the same (your) garden. Ok, granted, with some slicing operations you would have more escher-like windows but a metaphor always lacks some details :-)