how to transform numpy.float64 into str

You can use the `astype()` method of a NumPy array to change the data type of the array elements. To convert a `numpy.float64` to a string, you can use the `astype()` method with the argument `numpy.str_`.

Here's an example code snippet that demonstrates how to do this:

import numpy as np

# Create a NumPy array of float64 values
arr = np.array([3.14, 2.718, 1.618])

# Convert the float64 values to strings
str_arr = arr.astype(np.str_)

# Print the original and converted arrays
print("Original array:", arr)
print("Converted array:", str_arr)

This will output:

Original array: [3.14  2.718 1.618]
Converted array: ['3.14' '2.718' '1.618']

As you can see, the `astype()` method converts each float64 value in the array to a string. The resulting array has the data type `numpy.str_`.