Variables in Python

variable Assignment: variable assignment does not require a declaration type, each variable must be assigned before it is used, and the variable will not be created until the variable is assigned.

Multiple variable assignment: a=b=c=1 a,b,c value is 1

Specify multiple variables A, b, C = 1, 2 for multiple objects, "John" A value is 1, b value is 2, and C value is "John"

data type : Python has 5 data types numbers numbers, string strings, list lists, tuple tuples, dictionary dictionary numeric data classes are immutable data types, changing digital data The type assigns a new object.

There are 4 types of numbers: int, long, float, complex (plural, real and imaginary are floating-point)

Summary : Data types are divided into digital and non-digital.

Digital type includes integral type, long integer type, floating point type, plural type;

Non-numeric types include strings, lists, tuples, and dictionaries;

Non-digital similarities: You can use slices, links (+), repetition (*), value (a[]) and other related operations;

Different points of non-digital type:

Lists can be assigned directly, tuples cannot be assigned values, and dictionaries are assigned values in the Dict[k]=v way.

Delete : You can use Del to delete a reference to an object. Grammar del Var1[,var2[,var3[....,varn]]

VAR1 = 1 The Number object is created when the specified value is #
VAR2 = 10

del var deletes a reference to a single object
Del var_a, var_b Delete references to multiple objects

string : There are two values in order from left to right index starting from 0, maximum range is string length-1

The right-to-left index starts at 1, and the maximum range is the beginning of the string

string interception : variable [head subscript: Tail subscript] subscript empty to take the head or tail

The result of str = ' Ilovepython ' s[1:5] is love. with head not including tail

Print str # output full string
Print Str[0] # The first character in the output string
Print Str[2:5] # The string between the third and fifth in the output string
Print str[2:] # Outputs a string starting from the third character
Print str * 2 # output String two times Hello world! Hello world!
Print str + "TEST" # Output concatenated string Hello world! TEST

The plus sign (+) is a string join operator, and an asterisk (*) is a repeating operation.

List : The most common type of composite data is Python, which uses the most frequent data types and the list is identified with [].

The list can contain characters, numbers, strings, lists (that is, nested)

interception of lists: methods are the same as string interception. variable [head subscript: Tail subscript] subscript empty to take the head or tail

list = [' Runoob ', 786, 2.23, ' John ', 70.2]
Tinylist = [123, ' John ']

Print List # output complete listing [' Runoob ', 786, 2.23, ' John ', 70.2]
Print List[0] # The first element of the output list Runoob
Print List[1:3] # outputs the second to third element [786, 2.23]
Print list[2:] # Outputs all elements from the third start to the end of the list [2.23, ' John ', 70.2]
Print tinylist * 2 # Output List two times [123, ' John ', 123, ' John ']
Print List + tinylist # printing combinations [' Runoob ', 786, 2.23, ' John ', 70.2, 123, ' John ']

tuples : Tuples are similar to lists, with () identifiers, inner elements separated by commas, but tuples cannot be assigned two times, equivalent to read-only lists

the interception of tuples : Methods and strings, as well as list methods. variable [head subscript: Tail subscript] subscript empty to take the head or tail

tuple = (' Runoob ', 786, 2.23, ' John ', 70.2)
Tinytuple = (123, ' John ')

Print Tuple # output complete tuple (' Runoob ', 786, 2.23, ' John ', 70.2)
Print Tuple[0] # The first element of an output tuple Runoob
Print Tuple[1:3] # outputs the second to third element (786, 2.23)
Print tuple[2:] # Outputs all elements from the third start to the end of the list (2.23, ' John ', 70.2)
Print Tinytuple * 2 # Output tuple two times (123, ' John ', 123, ' John ')
Print tuple + tinytuple # Printing Group of tuples (' Runoob ', 786, 2.23, ' John ', 70.2, 123, ' John ')

list allows updates, tuples do not allow updates: tuple = (' Runoob ', 786, 2.23, ' John ', 70.2)
list = [' Runoob ', 786, 2.23, ' John ', 70.2]
Tuple[2] = 1000 # tuples are illegal to apply
List[2] = 1000 # is a legitimate application in the list

Dictionary : The most flexible built-in data structure type except for the list. A list is an ordered collection of objects, and a dictionary is an unordered collection of objects.

The difference is that the elements in the dictionary are accessed by keys rather than by offsets.

The dictionary is identified with "{}". A dictionary consists of an index (key) and a value corresponding to it.

Dict = {}
dict[' One ' = "This is one"
DICT[2] = "This is the"

tinydict = {' name ': ' John ', ' Code ': 6734, ' dept ': ' Sales '} ortinydict = {' name ': ' John ', ' Code ': 6734, 2: ' Sales '} or Dict = {1: ' This ', 2: ' Is ', 3: ' Dictionary ', 4:4}

Print dict[' One ' # output key for ' One ' value this is one
Print dict[2] # output key is a value of 2 this is the
Print Tinydict # Output full dictionary {' dept ': ' Sales ', ' Code ': 6734, ' name ': ' John '}
Print Tinydict.keys () # Output all keys [' dept ', ' Code ', ' name ']
Print tinydict.values () # Output all values [' Sales ', 6734, ' John ']

Conversion of data types : Simply use the data type as a function name.

All data types in Python are classes and can be viewed by type () to see data types such as N=1 type (n)

or Isinstance to judge.such as a = 111
Isinstance (A, int)
Returns True

Difference: type () does not consider a subclass to be a parent class type.
Isinstance () considers a subclass to be a parent class type.

Class A:
... pass
...
>>> class B (A):
... pass
...
>>> Isinstance (A (), a)
True
>>> type (A ()) = = A
False
>>> isinstance (B (), A)
True
>>> type (B ()) = = A
False

Variables in Python