Does the for/in loop construct preserve order?

Question

Does an ordinary for/in statement guarantee the list is iterated in order?

my_list = [5,4,3,2] for i in my_list     print(i) 

That is, is the loop above guaranteed to print 5 4 3 2 every time?

Answer 1

A for loop's iteration order is controlled by whatever object it's iterating over. Iterating over an ordered collection like a list is guaranteed to iterate over elements in the list's order, but iterating over an unordered collection like a set makes almost no order guarantees.

Answer 2

When you iterate over a sequence (list, tuple, etc.), the order is guaranteed. Hashed structures (dict, set, etc.) have their own order -- but for a given structure, the order will be the same each time. If you add or delete an element, the order may then be different.

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Consider the folloing code: I make a set of five elements, and then print it out with four identical for loops. The order is the same. Then I add two elements; this upsets the order.

my_set = set(["Apple", "Banana", "Casaba", "Dinner", "Eggplant"])  for food in my_set:     print food, print "\n"  for food in my_set:     print food, print "\n"  for food in my_set:     print food, print "\n"  for food in my_set:     print food, print "\n"  my_set.add("Fruitcacke") my_set.add("Grape")  for food in my_set:     print food, print "\n" 

Output:

Casaba Dinner Apple Eggplant Banana   Casaba Dinner Apple Eggplant Banana   Casaba Dinner Apple Eggplant Banana   Casaba Dinner Apple Eggplant Banana   Casaba Fruitcacke Grape Apple Dinner Eggplant Banana  

Note how the original elements are no longer in the same order: "Dinner" now comes after "Apple".