Strings in a DataFrame, but dtype is object

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The dtype object comes from NumPy, it describes the type of element in a ndarray. Every element in an ndarray must have the same size in bytes. For int64 and float64, they are 8 bytes. But for strings, the length of the string is not fixed. So instead of saving the bytes of strings in the ndarray directly, Pandas uses an object ndarray, which saves pointers to objects; because of this the dtype of this kind ndarray is object.

Here is an example:

  • the int64 array contains 4 int64 value.
  • the object array contains 4 pointers to 3 string objects.

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