List Methods

Книга: Practical Programming, Fourth Edition

Lists are objects and thus have methods. Table 6, , gives some of the most commonly used list methods.

Table 6. List Methods

Method	Description
L.append	Appends value v to list L
L.clear	Removes all items from list L
L.count	Returns the number of occurrences of v in list L
L.extend	Appends the items in v to L
L.index	Returns the index of the first occurrence of v in the sublist L[start:stop]. An error is reported if v doesn’t occur in the sublist.
L.insert	Inserts value v at index i in list L, shifting subsequent items to make room
L.pop	Removes and returns the last item of L (which must be nonempty)
L.remove	Removes the first occurrence of value v from list L
L.reverse	Reverses the order of the values in list L
L.sort	Sorts the values in list L in ascending order (numerical for numbers, alphabetical for strings). The reverse flag can be set to sort in descending order.

Here is a sample interaction that demonstrates how you can use list methods to create a list of various colors:

	>>> colors = ['red', 'orange', 'green']
	>>> colors.extend(['black', 'blue'])
	>>> colors
	['red', 'orange', 'green', 'black', 'blue']
	>>> colors.append('purple')
	>>> colors
	['red', 'orange', 'green', 'black', 'blue', 'purple']
	>>> colors.insert(2, 'yellow')
	>>> colors
	['red', 'orange', 'yellow', 'green', 'black', 'blue', 'purple']
	>>> colors.remove('black')
	>>> colors
	['red', 'orange', 'yellow', 'green', 'blue', 'purple']

All the methods shown here modify the list instead of creating a new list. The same is true for the methods clear, reverse, sort, and pop. Of those methods, only pop returns a value other than None: specifically, the item that was removed from the list. The only method that returns a list is copy, which is equivalent to L[:].

Finally, a call to append isn’t the same as using the plus sign (+). First, append appends a single value, while + expects two lists as operands. Second, append modifies the list rather than creating a new one.

Programmers occasionally forget that many list methods return None rather than creating and returning a new list. As a result, lists sometimes seem to disappear:

	>>> colors = 'red orange yellow green blue purple'.split()
	>>> colors
	['red', 'orange', 'yellow', 'green', 'blue', 'purple']
	>>> sorted_colors = colors.sort()
	>>> print(sorted_colors)
	None

In this example, colors.sort performed two actions: it sorted the items in the list and returned the value None. That’s why variable sorted_colors refers to None. Variable colors, on the other hand, refers to the sorted list:

	>>> colors
	['blue', 'green', 'orange', 'purple', 'red', 'yellow']

Methods that mutate a collection, such as append and sort, return None; it’s a typical error to expect that they’ll return the resulting list. As discussed in , mistakes like these can be caught by writing and running a few tests.

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