Sebastian Raschka, 03/2014
Code was executed in Python 3.4.0
True and False in the datetime modulePointed out in a nice article "A false midnight" at http://lwn.net/SubscriberLink/590299/bf73fe823974acea/:
-"it often comes as a big surprise for programmers to find (sometimes by way of a hard-to-reproduce bug) that,
unlike any other time value, midnight (i.e. datetime.time(0,0,0)) is False.
A long discussion on the python-ideas mailing list shows that, while surprising,
that behavior is desirable—at least in some quarters."
import datetime
-
-print('"datetime.time(0,0,0)" (Midnight) evaluates to', bool(datetime.time(0,0,0)))
-
-print('"datetime.time(1,0,0)" (1 am) evaluates to', bool(datetime.time(1,0,0)))
-Truemy_true_val = True
-
-
-print('my_true_val == True:', my_true_val == True)
-print('my_true_val is True:', my_true_val is True)
-
-print('my_true_val == None:', my_true_val == None)
-print('my_true_val is None:', my_true_val is None)
-
-print('my_true_val == False:', my_true_val == False)
-print('my_true_val is False:', my_true_val is False)
-
-print(my_true_val
-if my_true_val:
- print('"if my_true_val:" is True')
-else:
- print('"if my_true_val:" is False')
-
-if not my_true_val:
- print('"if not my_true_val:" is True')
-else:
- print('"if not my_true_val:" is False')
-Falsemy_false_val = False
-
-
-print('my_false_val == True:', my_false_val == True)
-print('my_false_val is True:', my_false_val is True)
-
-print('my_false_val == None:', my_false_val == None)
-print('my_false_val is None:', my_false_val is None)
-
-print('my_false_val == False:', my_false_val == False)
-print('my_false_val is False:', my_false_val is False)
-
-
-if my_false_val:
- print('"if my_false_val:" is True')
-else:
- print('"if my_false_val:" is False')
-
-if not my_false_val:
- print('"if not my_false_val:" is True')
-else:
- print('"if not my_false_val:" is False')
-None 'value'my_none_var = None
-
-print('my_none_var == True:', my_none_var == True)
-print('my_none_var is True:', my_none_var is True)
-
-print('my_none_var == None:', my_none_var == None)
-print('my_none_var is None:', my_none_var is None)
-
-print('my_none_var == False:', my_none_var == False)
-print('my_none_var is False:', my_none_var is False)
-
-
-if my_none_var:
- print('"if my_none_var:" is True')
-else:
- print('"if my_none_var:" is False')
-
-if not my_none_var:
- print('"if not my_none_var:" is True')
-else:
- print('"if not my_none_var:" is False')
-my_empty_string = ""
-
-print('my_empty_string == True:', my_empty_string == True)
-print('my_empty_string is True:', my_empty_string is True)
-
-print('my_empty_string == None:', my_empty_string == None)
-print('my_empty_string is None:', my_empty_string is None)
-
-print('my_empty_string == False:', my_empty_string == False)
-print('my_empty_string is False:', my_empty_string is False)
-
-
-if my_empty_string:
- print('"if my_empty_string:" is True')
-else:
- print('"if my_empty_string:" is False')
-
-if not my_empty_string:
- print('"if not my_empty_string:" is True')
-else:
- print('"if not my_empty_string:" is False')
-It is generally not a good idea to use the == to check for empty lists...
my_empty_list = []
-
-
-print('my_empty_list == True:', my_empty_list == True)
-print('my_empty_list is True:', my_empty_list is True)
-
-print('my_empty_list == None:', my_empty_list == None)
-print('my_empty_list is None:', my_empty_list is None)
-
-print('my_empty_list == False:', my_empty_list == False)
-print('my_empty_list is False:', my_empty_list is False)
-
-
-if my_empty_list:
- print('"if my_empty_list:" is True')
-else:
- print('"if my_empty_list:" is False')
-
-if not my_empty_list:
- print('"if not my_empty_list:" is True')
-else:
- print('"if not my_empty_list:" is False')
-
-
-
-my_zero_list = [0]
-
-
-print('my_zero_list == True:', my_zero_list == True)
-print('my_zero_list is True:', my_zero_list is True)
-
-print('my_zero_list == None:', my_zero_list == None)
-print('my_zero_list is None:', my_zero_list is None)
-
-print('my_zero_list == False:', my_zero_list == False)
-print('my_zero_list is False:', my_zero_list is False)
-
-
-if my_zero_list:
- print('"if my_zero_list:" is True')
-else:
- print('"if my_zero_list:" is False')
-
-if not my_zero_list:
- print('"if not my_zero_list:" is True')
-else:
- print('"if not my_zero_list:" is False')
-List comparisons are a handy way to show the difference between == and is.
While == is rather evaluating the equality of the value, is is checking if two objects are equal. The examples below show that we can assign a pointer to the same list object by using =, e.g., list1 = list2.
a) If we want to make a shallow copy of the list values, we have to make a little tweak: list1 = list2[:], or
b) a deepcopy via list1 = copy.deepcopy(list2)
Possibly the best explanation of shallow vs. deep copies I've read so far:
-*** "Shallow copies duplicate as little as possible. A shallow copy of a collection is a copy of the collection structure, not the elements. With a shallow copy, two collections now share the individual elements. Deep copies duplicate everything. A deep copy of a collection is two collections with all of the elements in the original collection duplicated."***
-(via S.Lott on StackOverflow)
-List modification of the original list doesn't affect
shallow copies or deep copies if the list contains literals.
from copy import deepcopy
-
-my_first_list = [1]
-my_second_list = [1]
-print('my_first_list == my_second_list:', my_first_list == my_second_list)
-print('my_first_list is my_second_list:', my_first_list is my_second_list)
-
-my_third_list = my_first_list
-print('my_first_list == my_third_list:', my_first_list == my_third_list)
-print('my_first_list is my_third_list:', my_first_list is my_third_list)
-
-my_shallow_copy = my_first_list[:]
-print('my_first_list == my_shallow_copy:', my_first_list == my_shallow_copy)
-print('my_first_list is my_shallow_copy:', my_first_list is my_shallow_copy)
-
-my_deep_copy = deepcopy(my_first_list)
-print('my_first_list == my_deep_copy:', my_first_list == my_deep_copy)
-print('my_first_list is my_deep_copy:', my_first_list is my_deep_copy)
-
-print('\nmy_third_list:', my_third_list)
-print('my_shallow_copy:', my_shallow_copy)
-print('my_deep_copy:', my_deep_copy)
-
-my_first_list[0] = 2
-print('after setting "my_first_list[0] = 2"')
-print('my_third_list:', my_third_list)
-print('my_shallow_copy:', my_shallow_copy)
-print('my_deep_copy:', my_deep_copy)
-List modification of the original list does affect
shallow copies, but not deep copies if the list contains compound objects.
my_first_list = [[1],[2]]
-my_second_list = [[1],[2]]
-print('my_first_list == my_second_list:', my_first_list == my_second_list)
-print('my_first_list is my_second_list:', my_first_list is my_second_list)
-
-my_third_list = my_first_list
-print('my_first_list == my_third_list:', my_first_list == my_third_list)
-print('my_first_list is my_third_list:', my_first_list is my_third_list)
-
-my_shallow_copy = my_first_list[:]
-print('my_first_list == my_shallow_copy:', my_first_list == my_shallow_copy)
-print('my_first_list is my_shallow_copy:', my_first_list is my_shallow_copy)
-
-my_deep_copy = deepcopy(my_first_list)
-print('my_first_list == my_deep_copy:', my_first_list == my_deep_copy)
-print('my_first_list is my_deep_copy:', my_first_list is my_deep_copy)
-
-print('\nmy_third_list:', my_third_list)
-print('my_shallow_copy:', my_shallow_copy)
-print('my_deep_copy:', my_deep_copy)
-
-my_first_list[0][0] = 2
-print('after setting "my_first_list[0][0] = 2"')
-print('my_third_list:', my_third_list)
-print('my_shallow_copy:', my_shallow_copy)
-print('my_deep_copy:', my_deep_copy)
-a = 1
-b = 1
-print('a is b', bool(a is b))
-True
-
-a = 999
-b = 999
-print('a is b', bool(a is b))
-