118: Python basics – saving python objects to disk with pickle

Sometimes we may wish to save Python objects to disc (for example if we have performed a lot of processing to get to a certain point). We can use Python’s pickle method to save and reload any Python object. Here we will save and reload a NumPy array, and then save and reload a collection of different objects.

Saving a single python object

Here we will use pickle to save a single object, a NumPy array.

import pickle 
import numpy as np

# Create array of random numbers:
my_array= np.random.rand(2,4)
print (my_array)

Out:

[[0.6383297  0.45250192 0.09882854 0.84896196]
 [0.97006917 0.29206495 0.92500062 0.52965801]]

# Save using pickle
filename = 'pickled_array.p'
with open(filename, 'wb') as filehandler:
    pickle.dump(my_array, filehandler)

Reload and print pickled array:

filename = 'pickled_array.p'
with open(filename, 'rb') as filehandler: 
    reloaded_array = pickle.load(filehandler)

print ('Reloaded array:')
print (reloaded_array)

Out:

Reloaded array:
[[0.6383297  0.45250192 0.09882854 0.84896196]
 [0.97006917 0.29206495 0.92500062 0.52965801]]

Using a tuple to save multiple objects

We can use pickle to save a collection of objects grouped together as a list, a dictionary, or a tuple. Here we will save a collection of objects as a tuple.

# Create an array, a list, and a dictionary
my_array = np.random.rand(2,4)
my_list =['A', 'B', 'C']
my_dictionary = {'name': 'Bob', 'Age': 42}

# Save all items in a tuple
items_to_save = (my_array, my_list, my_dictionary)
filename = 'pickled_tuple_of_objects.p'
with open(filename, 'wb') as filehandler:
    pickle.dump(items_to_save, filehandler)

Reload pickled tuple, unpack the objects, and print them.

filename = 'pickled_tuple_of_objects.p'
with open(filename, 'rb') as filehandler:
    reloaded_tuple = pickle.load(filehandler)


reloaded_array = reloaded_tuple[0]
reloaded_list = reloaded_tuple[1]
reloaded_dict = reloaded_tuple[2]

print ('Reloaded array:')
print (reloaded_array)
print ('\nReloaded list:')
print (reloaded_list)
print ('\n Reloaded dictionary')
print (reloaded_dict)

Out:

Reloaded array:
[[0.40193978 0.55173167 0.89411291 0.84625061]
 [0.86540981 0.27835353 0.43359222 0.31579122]]

Reloaded list:
['A', 'B', 'C']

 Reloaded dictionary
{'name': 'Bob', 'Age': 42}