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Python Tutorial |  |
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This chapter describes some things you've learned about already in more detail, and adds some new things as well.
The list data type has some more methods. Here are all of the methods of list objects:
append(x)
a[len(a):] = [x].
extend(L)
a[len(a):] = L.
insert(i, x)
a.insert(0, x) inserts at
the front of the list, and a.insert(len(a), x) is equivalent to
a.append(x).
remove(x)
x.
It is an error if there is no such item.
pop([i])
a.pop() returns the last item in the
list. The item is also removed from the list.
index(x)
x.
It is an error if there is no such item.
count(x)
x appears in the list.
sort()
reverse()
An example that uses most of the list methods:
>>> a = [66.6, 333, 333, 1, 1234.5]
>>> print a.count(333), a.count(66.6), a.count('x')
2 1 0
>>> a.insert(2, -1)
>>> a.append(333)
>>> a
[66.6, 333, -1, 333, 1, 1234.5, 333]
>>> a.index(333)
1
>>> a.remove(333)
>>> a
[66.6, -1, 333, 1, 1234.5, 333]
>>> a.reverse()
>>> a
[333, 1234.5, 1, 333, -1, 66.6]
>>> a.sort()
>>> a
[-1, 1, 66.6, 333, 333, 1234.5]
The list methods make it very easy to use a list as a stack, where the last element added is the first element retrieved (``last-in, first-out''). To add an item to the top of the stack, use append(). To retrieve an item from the top of the stack, use pop() without an explicit index. For example:
>>> stack = [3, 4, 5] >>> stack.append(6) >>> stack.append(7) >>> stack [3, 4, 5, 6, 7] >>> stack.pop() 7 >>> stack [3, 4, 5, 6] >>> stack.pop() 6 >>> stack.pop() 5 >>> stack [3, 4]
You can also use a list conveniently as a queue, where the first
element added is the first element retrieved (``first-in,
first-out''). To add an item to the back of the queue, use
append(). To retrieve an item from the front of the queue,
use pop() with 0 as the index. For example:
>>> queue = ["Eric", "John", "Michael"]
>>> queue.append("Terry") # Terry arrives
>>> queue.append("Graham") # Graham arrives
>>> queue.pop(0)
'Eric'
>>> queue.pop(0)
'John'
>>> queue
['Michael', 'Terry', 'Graham']
There are three built-in functions that are very useful when used with lists: filter(), map(), and reduce().
"filter(function, sequence)" returns a sequence (of
the same type, if possible) consisting of those items from the
sequence for which function(item) is true. For
example, to compute some primes:
>>> def f(x): return x % 2 != 0 and x % 3 != 0 ... >>> filter(f, range(2, 25)) [5, 7, 11, 13, 17, 19, 23]
"map(function, sequence)" calls
function(item) for each of the sequence's items and
returns a list of the return values. For example, to compute some
cubes:
>>> def cube(x): return x*x*x ... >>> map(cube, range(1, 11)) [1, 8, 27, 64, 125, 216, 343, 512, 729, 1000]
More than one sequence may be passed; the function must then have as
many arguments as there are sequences and is called with the
corresponding item from each sequence (or None if some sequence
is shorter than another). If None is passed for the function,
a function returning its argument(s) is substituted.
Combining these two special cases, we see that "map(None, list1, list2)" is a convenient way of turning a pair of lists into a list of pairs. For example:
>>> seq = range(8) >>> def square(x): return x*x ... >>> map(None, seq, map(square, seq)) [(0, 0), (1, 1), (2, 4), (3, 9), (4, 16), (5, 25), (6, 36), (7, 49)]
"reduce(func, sequence)" returns a single value constructed by calling the binary function func on the first two items of the sequence, then on the result and the next item, and so on. For example, to compute the sum of the numbers 1 through 10:
>>> def add(x,y): return x+y ... >>> reduce(add, range(1, 11)) 55
If there's only one item in the sequence, its value is returned; if the sequence is empty, an exception is raised.
A third argument can be passed to indicate the starting value. In this case the starting value is returned for an empty sequence, and the function is first applied to the starting value and the first sequence item, then to the result and the next item, and so on. For example,
>>> def sum(seq): ... def add(x,y): return x+y ... return reduce(add, seq, 0) ... >>> sum(range(1, 11)) 55 >>> sum([]) 0
There is a way to remove an item from a list given its index instead
of its value: the del statement. This can also be used to
remove slices from a list (which we did earlier by assignment of an
empty list to the slice). For example:
>>> a [-1, 1, 66.6, 333, 333, 1234.5] >>> del a[0] >>> a [1, 66.6, 333, 333, 1234.5] >>> del a[2:4] >>> a [1, 66.6, 1234.5]
del can also be used to delete entire variables:
>>> del a
Referencing the name a hereafter is an error (at least until
another value is assigned to it). We'll find other uses for
del later.
We saw that lists and strings have many common properties, e.g., indexing and slicing operations. They are two examples of sequence data types. Since Python is an evolving language, other sequence data types may be added. There is also another standard sequence data type: the tuple.
A tuple consists of a number of values separated by commas, for instance:
>>> t = 12345, 54321, 'hello!' >>> t[0] 12345 >>> t (12345, 54321, 'hello!') >>> # Tuples may be nested: ... u = t, (1, 2, 3, 4, 5) >>> u ((12345, 54321, 'hello!'), (1, 2, 3, 4, 5))
As you see, on output tuples are alway enclosed in parentheses, so that nested tuples are interpreted correctly; they may be input with or without surrounding parentheses, although often parentheses are necessary anyway (if the tuple is part of a larger expression).
Tuples have many uses, e.g., (x, y) coordinate pairs, employee records from a database, etc. Tuples, like strings, are immutable: it is not possible to assign to the individual items of a tuple (you can simulate much of the same effect with slicing and concatenation, though).
A special problem is the construction of tuples containing 0 or 1 items: the syntax has some extra quirks to accommodate these. Empty tuples are constructed by an empty pair of parentheses; a tuple with one item is constructed by following a value with a comma (it is not sufficient to enclose a single value in parentheses). Ugly, but effective. For example:
>>> empty = ()
>>> singleton = 'hello', # <-- note trailing comma
>>> len(empty)
0
>>> len(singleton)
1
>>> singleton
('hello',)
The statement t = 12345, 54321, 'hello!' is an example of
tuple packing: the values 12345, 54321 and
'hello!' are packed together in a tuple. The reverse operation
is also possible, e.g.:
>>> x, y, z = t
This is called, appropriately enough, tuple unpacking. Tuple unpacking requires that the list of variables on the left have the same number of elements as the length of the tuple. Note that multiple assignment is really just a combination of tuple packing and tuple unpacking!
Occasionally, the corresponding operation on lists is useful: list unpacking. This is supported by enclosing the list of variables in square brackets:
>>> a = ['spam', 'eggs', 100, 1234] >>> [a1, a2, a3, a4] = a
Another useful data type built into Python is the dictionary.
Dictionaries are sometimes found in other languages as ``associative
memories'' or ``associative arrays''. Unlike sequences, which are
indexed by a range of numbers, dictionaries are indexed by keys,
which can be any immutable type; strings and numbers can always be
keys. Tuples can be used as keys if they contain only strings,
numbers, or tuples. You can't use lists as keys, since lists can be
modified in place using their append() method.
It is best to think of a dictionary as an unordered set of
key:value pairs, with the requirement that the keys are unique
(within one dictionary).
A pair of braces creates an empty dictionary: {}.
Placing a comma-separated list of key:value pairs within the
braces adds initial key:value pairs to the dictionary; this is also the
way dictionaries are written on output.
The main operations on a dictionary are storing a value with some key
and extracting the value given the key. It is also possible to delete
a key:value pair
with del.
If you store using a key that is already in use, the old value
associated with that key is forgotten. It is an error to extract a
value using a non-existent key.
The keys() method of a dictionary object returns a list of all
the keys used in the dictionary, in random order (if you want it
sorted, just apply the sort() method to the list of keys). To
check whether a single key is in the dictionary, use the
has_key() method of the dictionary.
Here is a small example using a dictionary:
>>> tel = {'jack': 4098, 'sape': 4139}
>>> tel['guido'] = 4127
>>> tel
{'sape': 4139, 'guido': 4127, 'jack': 4098}
>>> tel['jack']
4098
>>> del tel['sape']
>>> tel['irv'] = 4127
>>> tel
{'guido': 4127, 'irv': 4127, 'jack': 4098}
>>> tel.keys()
['guido', 'irv', 'jack']
>>> tel.has_key('guido')
1
The conditions used in while and if statements above can
contain other operators besides comparisons.
The comparison operators in and not in check whether a value
occurs (does not occur) in a sequence. The operators is and
is not compare whether two objects are really the same object; this
only matters for mutable objects like lists. All comparison operators
have the same priority, which is lower than that of all numerical
operators.
Comparisons can be chained: e.g., a < b == c tests whether
a is less than b and moreover b equals c.
Comparisons may be combined by the Boolean operators and and
or, and the outcome of a comparison (or of any other Boolean
expression) may be negated with not. These all have lower
priorities than comparison operators again; between them, not has
the highest priority, and or the lowest, so that
A and not B or C is equivalent to (A and (not B)) or C. Of
course, parentheses can be used to express the desired composition.
The Boolean operators and and or are so-called
shortcut operators: their arguments are evaluated from left to
right, and evaluation stops as soon as the outcome is determined.
E.g., if A and C are true but B is false, A
and B and C does not evaluate the expression C. In general, the
return value of a shortcut operator, when used as a general value and
not as a Boolean, is the last evaluated argument.
It is possible to assign the result of a comparison or other Boolean expression to a variable. For example,
>>> string1, string2, string3 = '', 'Trondheim', 'Hammer Dance' >>> non_null = string1 or string2 or string3 >>> non_null 'Trondheim'
Note that in Python, unlike C, assignment cannot occur inside expressions.
C programmers may grumble about this, but it avoids a common class of
problems encountered in C programs: typing = in an expression when
== was intended.
Sequence objects may be compared to other objects with the same sequence type. The comparison uses lexicographical ordering: first the first two items are compared, and if they differ this determines the outcome of the comparison; if they are equal, the next two items are compared, and so on, until either sequence is exhausted. If two items to be compared are themselves sequences of the same type, the lexicographical comparison is carried out recursively. If all items of two sequences compare equal, the sequences are considered equal. If one sequence is an initial subsequence of the other, the shorted sequence is the smaller one. Lexicographical ordering for strings uses the ASCII ordering for individual characters. Some examples of comparisons between sequences with the same types:
(1, 2, 3) < (1, 2, 4)
[1, 2, 3] < [1, 2, 4]
'ABC' < 'C' < 'Pascal' < 'Python'
(1, 2, 3, 4) < (1, 2, 4)
(1, 2) < (1, 2, -1)
(1, 2, 3) == (1.0, 2.0, 3.0)
(1, 2, ('aa', 'ab')) < (1, 2, ('abc', 'a'), 4)
Note that comparing objects of different types is legal. The outcome is deterministic but arbitrary: the types are ordered by their name. Thus, a list is always smaller than a string, a string is always smaller than a tuple, etc. Mixed numeric types are compared according to their numeric value, so 0 equals 0.0, etc.5.1