1. object and Variable
every thing is an object, variable is a name of an object.
1.1 object
each object has: type, value(content), id.
you can use type() functiong to find the type of an object.
type(3)
int
type(3.14)
float
type("hwdong.net")
str
type(False)
bool
the data type of these objects are different.
Each object has a unique ID, it is the address of that object in memory。
you can use id() to find the id of an object.
id(3)
140721594999248
id(3.14)
2830338663824
id(False)
140721594476912
id("hwdong.net")
2830341107888
you can use hex() convert id to hex format:
hex(id(3.14))
'0x292fd780030'
1.2 variable
You can give an object a name,this name is called variable. Then the variable refer to the object.
a = 2
pi = 3.14
my_homepage = "hwdong.net"
print(a)
print(pi)
print(my_homepage)
2
3.14
hwdong.net
The name of varialbe can changed to other object anytime.
print(id(a))
print(id(pi))
a = pi
print(id(a)) # a and pi all refer to the object 3.14
140721594999216
2830340981840
2830340981840
1.3 date type
Data types represent a kind of value which determines what operations can be performed on that data.
Numeric, non-numeric and Boolean (true/false) data are the most used data types.
Numeric
- int: represent a positive or negative integet number.
- float: represent a real number,aften called floating point real number
- complex: represent a complex number,such as: 2+3j
type(2+3j)
complex
Boolean:
- bool: onlye two value: True,False
Sequence Type
A sequence is an ordered collection of similar or different data types. Python has the following built-in sequence data types:
- str: represetn a string. A string value is a collection of one or more characters put in single, double or triple quotes.
- list: a list object is an ordered collection of one or more data items, not necessarily of the same type, put in square brackets.
- tuple: a tuple object is an ordered collection of one or more data items, not necessarily of the same type, put in parentheses.
type([1,2,3,4,5])
list
type((1,2,3,4,5))
tuple
you can use index to access the element of a sequence object. Index start from 0 to n-1 (where n is length of the object, the number of elements)
s = "hwdong.net"
l = [1,2,3,4,5]
t = (1,2,3,4,5)
print(s[1])
print(l[1])
print(t[1])
w
2
2
you can use len() to find the length of a sequence object:
print(len(s),len(l),len(t))
10 5 5
you can use negtive integer as index. the negtive index should be from -1 to -n.
print(s[-1])
print(l[-1])
print(t[-1])
t
5
5
Dictionary
- dict: A dictionary object is an unordered collection of data in a key:value pair form. A collection of such pairs is enclosed in curly brackets.
you can sue key to find the value cooresponding to the key:
d = {"john":32223426,"wang":453451098}
d["john"]
32223426
String
String literals are written by enclosing a sequence of characters in single quotes (‘hwdong’), double quotes (“hwdong”) or triple quotes (‘'’hwdong’’’ or “"”hwdong”””).
To represent the single quote in a string enclosed by quotes, You can use escape sequences to represent a special character. An escape sequences start with a escape character: backslash \。
For example, \" represent the character ",\\represent the character \. Following are other escape sequences
- \a Bell or alert. result::Bell sound
- \b Backspace, “ab\bc” result:: ac
- \f Formfeed “hello\fworld” result::hello world
- \n Newline “hello\nworld” result:Hello
- \nnn Octal notation, where n is in the range 0-7. ‘\101’ c: A
- \t Tab ‘Hello\tPython’ result: Hello Python
- \xnn Hexadecimal notation, where n is in the range 0-9, a-f, or A-F. ‘\x41’ result:A
print("hello\"nworld")
print("ab\bc")
print("hello\tworld")
print("hello\nworld")
print("hello\fworld")
print("\x41")
hello"nworld
abc
hello world
hello
world
hello world
A
Python raw string is created by prefixing a string literal with ‘r’ or ‘R’. Python raw string treats backslash () as a literal character. This is useful when we want to have a string that contains backslash and don’t want it to be treated as an escape character.
print(r"hello\nworld")
hello\nworld
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