Python dictionary and Set learning Summary
- 2020-06-07 04:47:47
- OfStack
Mapping type:
Represents a collection of 1 arbitrary objects that can be indexed by another collection of almost arbitrary key values
Unlike sequences, maps are unordered and indexed by key
Any immutable object can be used as a dictionary key, such as a string, number, tuple, and so on
Lists, dictionaries, and tuples containing mutable objects cannot be used as keys
Referencing a key that does not exist throws an KeyError exception
1) the dictionary
dict { } An empty dictionary
{ key1:value1,key2:value2,... }
Dictionaries are also called associative arrays or hash tables in other programming languages;
Access to elements through keys; Disordered set; Variable-type containers, variable-length, heterogeneous, nested
Supported operations:
len(D) return D Number of projects in
D[k] return D In the key k The value of the
D[k] = x will D[k] The value of the set x
>>> d1 = {'x':1,'y':2,'z':3}
>>> d1['x']
1
>>> d1['z'] By key index
3
del D[k] from D Remove the D[k]
>>> del d1['x']
>>> d1
{'y': 2, 'z': 3}
k in D if k is D , returns the value of True
Supported methods:
D.clear() Clear all elements
D.copy() copy 1 A copy of the
>>> d1 = {'x':1,'y':2,'z':3}
>>> id(d1)
45320640
>>> d2 = d1.copy() Deep copy
>>> id(d2)
45997776
>>> d3 = d1 A shallow copy
>>> id(d3)
45320640 d1 , d3 Point to the same 1 Object, d2 Pointing to the other 1 object
D.get(k[,d]) Gets the value of the corresponding key and returns if none exists d( The default is empty )
>>> d1.get('y')
2
D.has_key(k) Whether a key value exists, returns True or False.( Only in the pyhton2 The use of )
D.items() convert (key,value) A list of tuples
>>> d1.items()
[('y', 2), ('x', 1), ('z', 3)]
>>> t1,t2,t3 = d1.items()
>>> t1
('y', 2)
>>> t2
('x', 1)
>>> t3
('z', 3)
>>> m1,m2 = {'x':1,'y':2}
>>> print m1
'y'
>>> print m2
'x' Keys, not values, are saved !!!
D.values() List of values
>>> d1.values()
[2, 1, 3]
D.keys() List of key
>>> d1.keys()
['y', 'x', 'z']
D.pop(k[,d]) Pops up the specified key value, or raises an exception if not specified
>>> d1.pop()
TypeError: pop expected at least 1 arguments, got 0
>>> d1.pop('x')
1
>>> d1
{'y': 2, 'z': 3}
D.popitem() Random popup
>>> d1.popitem()
('y', 2)
>>> d1.popitem()
('z', 3)
>>> d1.popitem()
KeyError: 'popitem(): dictionary is empty' Is a space - time exception
>>> d1
{ }
D.update(m) Merge the dictionary
>>> d1 = { 'x':1,'y':2,'z':3 }
>>> d2={'c':'hello','y':66}
>>> d1.update(d2)
>>> d1
{'y': 66, 'x': 1, 'c': 'hello', 'z': 3} If the key exists, it will be overwritten; if not, it will be added
D.iteritems() return 1 Three iterator objects
>>> d1 = { 'x':1,'y':2,'z':3 }
>>> i1 = d1.iteritems()
>>> i1.next() use next The method traverses each 1 An element
('y', 2)
>>> i1.next()
('x':1)
>>> i1.next()
('z':3)
>>> i1.next()
StopIteration There is no restart after the traversal
D.iterkeys() -> an iterator over the keys of D
>>> i2 = d1.iterkey()
>>> i2.next()
'y'
D.itervalues() -> an iterator over the values of D
>>> i3 = d1.iterkey()
>>> i3.next()
2
D.viewvalues() Returns a collection - like dictionary ( The value of )
>>> d1.viewvalues()
dict_values([2, 1, 3])
D.viewitems() -> a set-like object providing a view on D's items( Key/value pair )
>>> d1.viewitems()
dict_items([('y', 2), ('x', 1), ('z', 3)])
D.viewkeys() -> a set-like object providing a view on D's keys
>>> d1.viewkeys()
dict_keys(['y', 'x', 'z'])
>>> d2 = dict(x=1,y=2,z=3) Definition dictionary 1 Kind of way
>>> d2
{'y': 2, 'x': 1, 'z': 3}
Supplement: zip Returns a list of tuples
>>> zip('xyz','123')
[('x', '1'), ('y', '2'), ('z', '3')] 11 Corresponding generation list
>>> zip('xyzm','123')
[('x', '1'), ('y', '2'), ('z', '3')] The redundant items are discarded
>>> zip('xyz','123','qer')
[('x', '1', 'q'), ('y', '2', 'e'), ('z', '3', 'r')]
>>> dict(zip('xyz','123')) To construct a dictionary
{'y': '2', 'x': '1', 'z': '3'}2) set
Disordered arrangement, hashing;
Support for set relationship testing
Membership test:
in
not in
The iteration
Not supported: index, element fetch, slice
Type of collection: set() frozenset()
variable immutable
There is no specific syntax format and it can only be created through factory functions
Ex. :
>>> s1=set(1,2,3)
TypeError: set expected at most 1 arguments, got 3 Wrong way
>>> s1 = set([1,2,3]) Right way
>>> s1
set([1, 2, 3])
>>> type(s1)
setSupported methods and actions:
3) summary
How to get help:
Gets the properties and methods the object supports using: dir()
Specific use of a method: help(list.pop)
Gets the document string of the callable object: print obj.__doc__
Containers, types, objects:
1 , lists, tuples, dictionary literals can be distributed in multiple lines without newline characters, and finally 1 Two characters can be followed by a comma ( If empty, do not use )
2 , all objects have reference counts (sys In the module getrefcount methods );
>>> import sys
>>> s1
set([1, 2, 3])
>>> sys.getrefcount(s1) To view s1 Reference count of
3
3 , lists, and dictionaries all support two types of replication: shallow and deep; Deep replication is available copy In the module deepcopy() The implementation.
4 , Python All objects in the 1 ", which means that all objects named with an identifier have the same state. Thus, being able to name all objects can be processed directly as data;
5 , all sequences support iteration; ( An ordered set of nonnegative integers )
6 , operations and methods supported by all sequences:
s[i] The index s[i:j] slice
s[i:j:stride] Extended slice len(s)
min(s) max(s) sum(s)
all(s) All for true any(s) Any for true
s1 + s2: The connection s1 * N: repeat
Membership judgment:
obj in s1
obj not in s1
7 , operation of variable sequence:
s[index] = value Elements of the assignment
s[i:j] = t Slice assignment
s[i:j:stride] = t Extended slice assignment
del s[index] Element to delete
del s[i:j] Slicing to delete
del s[i:j:stride] Extended slice deletion
Reference counting and garbage collection:
All objects have reference counts
Assign objects 1 Two new names or put them in 1 The reference count is increased for each container
with del When a statement or variable is re-assigned, its reference count is reduced
sys.getrefcount() You can get the current reference count of an object
1 When the reference counter returns to zero, it is collected by the garbage collection mechanism