2.1 Python commonly used built-in objects

Everything in Python is an object,
Integers, real complex numbers, strings, lists, tuples, dictionaries, collections,
Objects such as zip, map, enumerate, filter, and functions and classes are also objects.
Python built-in objects are shown in Table 2-1

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2.1.1 Constants and variables

Table 2-1, column 3 is a valid Python constant except the last 4 rows
A constant refers to a literal value that does not need to be changed and cannot be changed. For example, a number 3 and a list [1,2,3] are constants.
Variable values can be changed
Python does not need to declare variable names and types in advance,
- Direct assignment can create any type of object variable.
- Variable value is variable, type can also be changed
The statement creates an integer variable x and assigns a value of 3.

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Create string variable x, assign value
- The previous integer variable x no longer exists.

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Create a list object x, assign [1,2,3], the previous string variable $x$ no longer exists
This also applies to tuples, dictionaries, collections and other objects of any type in Python, including objects of custom types.

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Python uses a value-based memory management model.
The execution process of the assignment statement:
- First calculate the value of the expression on the right side of the equal sign
- Find a location in memory to store the value in,
- Finally, create a variable and point to this memory address.
Variables do not store values directly,
- And the memory address or reference where the value is stored,
- This is why the variable type can be changed at any time

No need to explicitly declare variables and their types before use,
- But Python is an out-of-the-box, strongly typed programming language,
- The Python interpreter will automatically infer the variable type based on the value of the expression on the right side of the assignment operator.
The working mode is similar to "state machine", after the variable is created, unless the variable type is explicitly modified or the variable is deleted, the variable will keep the previous type

If the variable appears to the left of the assignment operator or compound assignment operator (such as + =, * =, etc.)
It means to create a variable or modify the value of the variable,
Otherwise it means referencing the value of the variable,
- This also applies to using subscripts to access variable sequences such as lists and dictionaries
- And elements in custom objects

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A lot did not write

In Python, there are no character constants and variables, only string type constants and variables, and a single character is also a string.
Single quotes, double quotes, triple single quotes, triple double quotes as delimiters to represent strings,
- And different delimiters can be nested between
Python3.x supports Chinese, Chinese and English letters are treated as one character,
- You can even use Chinese as a measurement name
Plus sign connection string, Python string also provides a lot of methods
- Find, replace, typeset
- Many built-in functions and standard library objects also support string operations. Chapter 7 introduces them in detail.

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Built-in functions are one of Python's built-in object types. They can be used directly without the need to import any additional modules. These built-in objects are encapsulated in the built-in module _ builtins__, implemented in C language
and optimized a lot, very fast running speed, Recommend to use first.
Built-in function dir () can view all built-in functions and built-in objects

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help (function name) can view the usage of a function.
You can also use help (module name) to view the help documentation of the module without importing the module, such as help ('math ").
In Table 2-5, the parameters in square brackets can be omitted.

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If you encounter unfamiliar functions, you can view the help through the built-in function help ().
Prioritize the use of built-in functions when programming,
- The built-in functions are not only mature and stable, but also relatively fast.

(1) bin (), oct (), hex () convert integers to binary, octal and hexadecimal forms, the parameters must be integers

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int converts other forms of numbers to integers,
- The parameters are integers, real numbers, fractions, or legal numeric strings,
- When the parameter is a numeric string, the second parameter base is also allowed to specify the base of the numeric string.
- base takes an integer of 0 or 2 ~ 36,
- 0 means conversion by hexadecimal implied by numeric string.

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The built-in class asci converts objects to ASCII representation, using escape characters to represent specific characters when necessary.

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Built-in classes to generate byte strings,
- Or convert the specified object to a specific encoded byte string.

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(3) list (), tuple (), dict (), set (), frozenset () convert other types of data into list tuples, dictionaries, variable sets and immutable sets, or create empty lists and empty elements Groups, empty dictionaries and empty collections.

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(4) Built-in function to judge the data type,
- Check the function parameters,
- You can avoid the wrong parameter type causing the function to crash or return unexpected results.

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Compute lists, tuples, or other iterable objects that contain a finite number of elements
sum () by default (can be changed by the start parameter) supports sequences or iterable objects containing numeric elements,
max () and min () require comparable sizes between elements in a sequence or iterable

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