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dtype
The data type or dtype is a special object containing the information (or metadata, data about data) the ndarray needs to interpret a chunk of memory as a particular type of data:
NumPy data types
you can use shorthand type code strings to create ndarray
arr = np.array([1.1, 2.2, 3.3], dtype='f8')
astype
You can explicitly convert or cast an array from one dtype to another using ndarray’s astype method.
Calling astype always creates a new array (a copy of the data), even if the new dtype is the same as the old dtype.
integer -> float
float -> integer
string -> float
Note: If casting were to fail for some reason (like a string that cannot be converted to float64), a ValueError will be raised.
Reference
Python for Data Analysis Second Edition