Словари в python (dict)

Join Two Sets

There are several ways to join two or more sets in Python.

You can use the method that returns a new set containing all items from both sets,
or the method that inserts all the items from one set into another:

Example

The method returns a new set with all items from both sets:

set1 = {«a», «b» , «c»}set2 = {1, 2, 3}
set3 = set1.union(set2)print(set3)

Example

The method inserts the items in set2 into set1:

set1 = {«a», «b» , «c»}set2 = {1, 2, 3}
set1.update(set2)print(set1)

Note: Both and
will exclude any duplicate items.

There are other methods that joins two sets and keeps ONLY the duplicates, or NEVER the duplicates,
check the full list of set methods in the bottom of this page.

Python NumPy

NumPy IntroNumPy Getting StartedNumPy Creating ArraysNumPy Array IndexingNumPy Array SlicingNumPy Data TypesNumPy Copy vs ViewNumPy Array ShapeNumPy Array ReshapeNumPy Array IteratingNumPy Array JoinNumPy Array SplitNumPy Array SearchNumPy Array SortNumPy Array FilterNumPy Random
Random Intro
Data Distribution
Random Permutation
Seaborn Module
Normal Distribution
Binomial Distribution
Poisson Distribution
Uniform Distribution
Logistic Distribution
Multinomial Distribution
Exponential Distribution
Chi Square Distribution
Rayleigh Distribution
Pareto Distribution
Zipf Distribution

NumPy ufunc
ufunc Intro
ufunc Create Function
ufunc Simple Arithmetic
ufunc Rounding Decimals
ufunc Logs
ufunc Summations
ufunc Products
ufunc Differences
ufunc Finding LCM
ufunc Finding GCD
ufunc Trigonometric
ufunc Hyperbolic
ufunc Set Operations

Rejected Alternatives

The << operator didn’t seem to get much support on Python-Ideas,
but no major objections either. Perhaps the strongest objection was
Chris Angelico’s comment

Another suggestion was to create a new operator <-. Unfortunately
this would be ambiguous, d <- e could mean d merge e or
d less-than minus e.

A dict.merged() method would avoid the need for an operator at
all. One subtlety is that it would likely need slightly different
implementations when called as an unbound method versus as a bound
method.

As an unbound method, the behavior could be similar to:

def merged(cls, *mappings, **kw):
    new = cls()  # Will this work for defaultdict?
    for m in mappings:
        new.update(m)
    new.update(kw)
    return new

As a bound method, the behavior could be similar to:

def merged(self, *mappings, **kw):
    new = self.copy()
    for m in mappings:
        new.update(m)
    new.update(kw)
    return new

Arguably, methods are more discoverable than operators.
The method could accept any number of positional and keyword
arguments, avoiding the inefficiency of creating temporary dicts.
Accepts sequences of (key, value) pairs like the update
method.
Being a method, it is easily to override in a subclass if you need
alternative behaviors such as «first wins», «unique keys», etc.

Would likely require a new kind of method decorator which combined
the behavior of regular instance methods and classmethod. It
would need to be public (but not necessarily a builtin) for those
needing to override the method. There is a
proof of concept.
It isn’t an operator. Guido discusses why operators are useful.
For another viewpoint, see Nick Coghlan’s blog post.

Remove Item

To remove an item in a set, use the , or the method.

Remove «banana» by using the
method:

thisset = {«apple», «banana», «cherry»}
thisset.remove(«banana»)
print(thisset)

Note: If the item to remove does not exist, will raise an error.

Remove «banana» by using the
method:

thisset = {«apple», «banana», «cherry»}
thisset.discard(«banana»)
print(thisset)

Note: If the item to remove does not exist, will
NOT raise an error.

You can also use the , method to remove
an item, but this method will remove the last item. Remember that sets
are unordered, so you will not know what item that gets removed.

The return value of the method is the
removed item.

Remove the last item by using the
method:

thisset = {«apple», «banana», «cherry»}
x =
thisset.pop()print(x)
print(thisset)

Note: Sets are unordered, so when using the method,
you will not know which item that gets removed.

The
method empties the set:

thisset = {«apple», «banana», «cherry»}
thisset.clear()
print(thisset)

Python NumPy

Операции

Поскольку словари представляют собой мапы (map), а не последовательности, к ним
нельзя применить конкатенацию или срезы.

К словарям можно применять стандартные операторы сравнения:

    <, <=, ==, !=, >=, >

Для того чтобы сделать проход по ключам словаря, используем for:

>>> table = {'Python': 'Guido van Rossum',
...          'Perl':     'Larry Wall',
...          'Tcl':      'John Ousterhout' }
>>> for lang in table:
...     print(lang, table)
..
>>> Tcl     John Ousterhout
>>> Python  Guido van Rossum
>>> Perl    Larry Wall

Словари хорошо подходят для хранения многомерных массивов или матриц:

>>> Matrix = {}
>>> Matrix = 88
>>> Matrix = 99
>>>
>>> X = 2; Y = 3; Z = 4
>>> Matrix
88
>>> Matrix
{(2, 3, 4): 88, (7, 8, 9): 99}

С помощью словарей можно хранить структурированную информацию в виде записей:

>>> man = {'name': 'Serg',
...        'jobs': ,
...        'web': 'www.iakovlev.org',
...        'home': {'city': 'Moscow', 'zip':129000}}
>>> man
Serg
>>> man
'writer'

Благоприятные, средние и худшие случаи

При вычислении временной сложности операции можно получить сложность на основе благоприятного, среднего или худшего случая.

Благоприятный случай. Как следует из названия, это сценарий, когда структуры данных и элементы в коллекции вместе с параметрами находятся в оптимальном состоянии. Например, мы хотим найти элемент в коллекции. Если этот элемент оказывается первым элементом коллекции, то это лучший сценарий для операции.

Средний случай. Определяем сложность на основе распределения значений входных данных.

Худший случай. Структуры данных и элементы в коллекции вместе с параметрами находятся в наиболее неоптимальном состоянии. Например, худший случай для операции, которой требуется найти элемент в большой коллекции в виде списка — когда искомый элемент находится в самом конце, а алгоритм перебирает коллекцию с самого начала.

Python NumPy

Python Dictionary Methods

Methods that are available with a dictionary are tabulated below. Some of them have already been used in the above examples.

Method	Description
clear()	Removes all items from the dictionary.
copy()	Returns a shallow copy of the dictionary.
fromkeys(seq)	Returns a new dictionary with keys from `seq` and value equal to `v` (defaults to ).
get(key)	Returns the value of the `key`. If the `key` does not exist, returns `d` (defaults to ).
items()	Return a new object of the dictionary’s items in (key, value) format.
keys()	Returns a new object of the dictionary’s keys.
pop(key)	Removes the item with the `key` and returns its value or `d` if `key` is not found. If `d` is not provided and the `key` is not found, it raises .
popitem()	Removes and returns an arbitrary item (key, value). Raises if the dictionary is empty.
setdefault(key)	Returns the corresponding value if the `key` is in the dictionary. If not, inserts the `key` with a value of `d` and returns `d` (defaults to ).
update()	Updates the dictionary with the key/value pairs from `other`, overwriting existing keys.
values()	Returns a new object of the dictionary’s values

Here are a few example use cases of these methods.

Output

{'Math': 0, 'English': 0, 'Science': 0}
('Math', 0)
('English', 0)
('Science', 0)

Delete Dictionary Elements

You can either remove individual dictionary elements or clear the entire contents of a dictionary. You can also delete entire dictionary in a single operation.

To explicitly remove an entire dictionary, just use the del statement. Following is a simple example −

#!/usr/bin/python

dict = {'Name': 'Zara', 'Age': 7, 'Class': 'First'}
del dict; # remove entry with key 'Name'
dict.clear();     # remove all entries in dict
del dict ;        # delete entire dictionary

print "dict: ", dict
print "dict: ", dict

This produces the following result. Note that an exception is raised because after del dict dictionary does not exist any more −

dict:
Traceback (most recent call last):
   File "test.py", line 8, in <module>
      print "dict: ", dict;
TypeError: 'type' object is unsubscriptable

Note − del() method is discussed in subsequent section.

Python NumPy

Accessing Values in Dictionary

To access dictionary elements, you can use the familiar square brackets along with the key to obtain its value. Following is a simple example −

#!/usr/bin/python3

dict = {'Name': 'Zara', 'Age': 7, 'Class': 'First'}
print ("dict: ", dict)
print ("dict: ", dict)

When the above code is executed, it produces the following result −

dict:  Zara
dict:  7

If we attempt to access a data item with a key, which is not a part of the dictionary, we get an error as follows −

#!/usr/bin/python3

dict = {'Name': 'Zara', 'Age': 7, 'Class': 'First'};
print ("dict: ", dict)

When the above code is executed, it produces the following result −

dict:
Traceback (most recent call last):
   File "test.py", line 4, in <module>
      print "dict: ", dict;
KeyError: 'Alice'

Removing Items

There are several methods to remove items from a dictionary:

Example

The method removes the item with the specified key name:

thisdict = {
  «brand»: «Ford»,
  «model»: «Mustang»,
  «year»: 1964
}thisdict.pop(«model»)
print(thisdict)

Example

The method removes the last
inserted item (in versions before 3.7, a random item is removed instead):

thisdict = {
  «brand»: «Ford»,
  «model»: «Mustang»,
  «year»: 1964
}thisdict.popitem()
print(thisdict)

Example

The keyword removes the item with the specified
key name:

thisdict = {
  «brand»: «Ford»,
  «model»: «Mustang»,
  «year»: 1964
}del thisdictprint(thisdict)

Example

The keyword can also delete the
dictionary completely:

thisdict = {
  «brand»: «Ford»,
  «model»: «Mustang»,
  «year»: 1964
}del thisdictprint(thisdict) #this will cause an error because «thisdict»
no longer exists.

Example

The method empties the
dictionary:

thisdict = {
  «brand»: «Ford»,
  «model»: «Mustang»,
  «year»: 1964
}thisdict.clear()print(thisdict)