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This article introduces the knowledge of "what is the role of compilers and interpreters in Python". In the operation of actual cases, many people will encounter such a dilemma, so let the editor lead you to learn how to deal with these situations. I hope you can read it carefully and be able to achieve something!

I. presentation of data

We all know that in real life, there are many ways to express numbers, the common ones are binary, octal, decimal and hexadecimal. We are all familiar with the decimal system, and we have all memorized the addition formula table, mainly using 09, these 10 Arabic numerals to construct the entire decimal system, in which the core rule is "every decimal into one", and the borrower is "borrow one to ten". So why is it that decimal production is used as the basic system no matter what country, history, and cultural level in the world? Because we have 10 fingers, which is the most convenient to break! We are naturally friendly to the decimal system.

What about computers? The computer is not human, and there are no 10 fingers to break, so it can't use the decimal system. So what system does it use? Binary! Binary is a number represented by the numbers 0 and 1, that is, in the shape of 010101010. Its cardinality is 2, the carry rule is "every two into one", and the borrowing rule is "borrow one when two".

Why do computers use binary as their own machine language, that is, the representation of data? Because the smallest computing unit of the computer is determined according to the high and low level of the switching state, it only has the concepts of on and off, high and low, which is replaced by two kinds of mathematics: 0 and 1. Similarly, in terms of physical storage, the track of the hard disk can only distinguish between punched and unpunched states, which are also 0 and 1. At the same time, binary is convenient for addition, subtraction and counting coding. Binary and decimal numbers are easy to convert to each other. Binary is convenient for logical judgment (yes or no), and logical judgment is usually in two states, which goes well with binary. Binary representation data also has the characteristics of strong anti-interference ability and high reliability, because when it is subjected to a certain degree of electromagnetic interference, as long as it can tell whether it is high or low, it is not important to distinguish between 0 and 1, which is born with anti-jamming ability in network signals.

However, in human-computer communication, binary system has a fatal weakness, the writing of numbers is particularly lengthy, and there is no human readability! For example, a decimal 100000 written in binary is 11000011010100000, which is several times longer, and can you read its decimal number from such a long binary number?

Computers cannot exist independently, nor can they create themselves at present. Everything about it, whether it is input or output, must be communicated with people. So the problem is, human beings can only read decimal and English, Chinese, etc., but computers can only 010101, as for English, Chinese is a book of heaven for it. So how do we communicate with computers? How to encode our English or Chinese into 1010101 that the computer can recognize?

Second, the development of programming language 1. Punched note

We already know that computers only understand machine language, that is, binary data representation, and any operation and coding on it will eventually be unified on this, but this is a sad story.

At first, in order for the computer to work the way we wanted, programmers had to write that the computer could read and understand the machine code that was executed directly, that is, 01010101, and that's what punched notes did. Zero and one are represented by whether they are punched or not. OK, the computer is fine, it can read without barrier, but the programmer is uncomfortable. A pat on the head can also figure out the problem. Easy to make mistakes, low efficiency, difficult to write, difficult to maintain. It may be a simple print "hello world", or it may require a note several meters long. This is simply a primitive society, with abhorrent inefficiency. There's a fire or something, just Over. The only advantage is that it can be executed directly without conversion, but this benefit can be completely ignored in terms of relative disadvantages.

two。 Assembly language

Assembly language is a low-level language of programmable devices, also known as symbolic language. In assembly language, mnemonics are used to replace the opcodes of machine instructions, and address symbols or labels are used to replace the address of instructions or operands. In different devices, assembly language corresponds to different machine language instruction sets and is converted into machine instructions through the assembly process. In other words, there is no direct portability between different platforms, it is platform-related, and the assembler you write on this hardware platform cannot be transferred to another set of hardware. Therefore, assembly language is usually used in low-level, hardware operation and demanding program optimization. Assembly language is widely used in drivers, embedded operating systems and real-time running programs. Compared with machine code, it is more inclined to human language habits, easier to write and read, that is, a little abstract symbol conceptualization, which greatly improves the efficiency of programming. However, it is still a low-level language, and there is still room for improvement.

The above paragraph doesn't matter. To put it simply, assembly language is a little more human-friendly than punched notes, and at least a few English abbreviations like ADD\ CALL\ MOV can be used. However, it sacrifices some performance and is still not friendly enough.

3. C language

There are actually many low-level languages before C, and we don't care about them. In order to make programming easier, more efficient, and smart computer programmers, step by step invented FORTRAN, BASIC, B and many other languages, and then in 1972 gave birth to the well-known, the most widely used, the most influential, still unshakable C language.

Why is the C language so powerful? In the final analysis, it is a sentence: directly operate the hardware! The same algorithm, using C language, is much more efficient than JAVA and other languages. Then one may ask, what is the ratio of C to assembly and machine code? C is definitely slow, but the efficiency difference between writing an assembler and writing a C program is even bigger. C language achieves a high balance in terms of human friendliness and underlying relevance. The two contradict each other and cannot have both at the same time.

What did C do? In fact, it is a bigger step forward in terms of human friendliness than before. It is convenient for humans, but the machines are confused! What do you mean by giving me so many characters? The machine only knows binary! So how is the C code executed? Then we have to ask for a compiler!

The compiler translates the code written by the programming language into binary machine code that the machine can execute or "understand".

In fact, whether we install JAVA, C, or Python, we mainly install the "compiler" of this programming language.

4. Python language

Decades ago, C language was a well-deserved high-level language representative, and it is still the second largest language in the list. However, in many areas, it is no longer applicable. Today's more mainstream languages are those that are quick to use, easy to understand and, to put it bluntly, low barriers, so that more people can enter the programmer industry. Making programming easier and faster is the development trend in the future. In other words, it is necessary to make programming languages closer to human languages and further away from machine languages.

Python is such a language. Its grammar is simple and clear, and it is closer to the habits of human use. As a dynamic interpretive language, people can pay more attention to the details of business logic when writing code, without paying too much attention to data type definition, program efficiency and so on.

Since they all speak "human language" that the machine does not understand, it must also need a Python "compiler". For the Python language, the "compiler" in a broad sense is called an interpreter.

III. Compiler and interpreter

Compiler / interpreter: a translator between a high-level language and a machine

It translates the code into binary machine code that can be executed by the machine, but the working principle and the translation process are different.

So what's the difference between the two?

Use a popular example to compare: we went to a restaurant and ordered eight dishes and one soup. The way of the compiler is that the cook will cook all the dishes for you and bring them to you together, as for where you eat, how you eat, whatever. The way the interpreter works is that the cook cooks a dish and serves it to you, and you eat it, and you have to eat it in a restaurant.

For more in-depth how compilers and interpreters work, please refer to the epic compilation principles, which has a nickname called the Dragon Book.

4. Types of Python interpreters

Python has several versions of the interpreter:

CPython: official version of the interpreter. This interpreter is developed in C language, so it is called CPython. CPython is the most widely used Python interpreter. What we usually say, download, discuss, and use is this interpreter.

Ipython: an interactive interpreter based on CPython, enhanced in the interactive way, the function of executing Python code is exactly the same as CPython. CPython uses > > as the prompt, while IPython uses In [serial number]: as the prompt.

PyPy: a Python interpreter that pursues execution speed. Using JIT technology, dynamic compilation of Python code (note, not interpretation) can significantly improve the execution speed of Python code. Most CPython code can be run under PyPy, but there are some differences, which results in different results for the same Python code executed under the two interpreters.

Jython: a Python interpreter running on the Java platform that can directly compile Python code into Java bytecode for execution.

IronPython: similar to Jython, except that IronPython is a Python interpreter running on Microsoft's .net platform that compiles Python code directly into .net bytecode.

Fifth, the operating mechanism of Python

As a dynamic interpretive language, the operation mechanism of Python can be seen in the following figure (the picture comes from the network, where the "compiler" is a broad term for the interpreter):

It is said that the interpreter is slow, but Python also thinks of ways to improve the running speed, that is, to use pyc files. This refers to JAVA's bytecode approach, but it's not exactly the same.

The code we write is usually saved in a file with the suffix .py. When the program is executed, the interpreter reads the source code line by line and runs it line by line. Each time it is executed, the process is repeated, which consumes a lot of repetitive interpretation work. In order to reduce this repetitive interpretation work, Python introduced the pyc file, and the pyc file is the file that saves the interpretation results of the py file, so that there is no need to explain the next time you run, just use the pyc file, which undoubtedly greatly improves the running speed of the program.

For pyc files, you must know the following:

The pyc file will not be generated for the currently called main program

The pyc file will be generated only if the module is imported into the main program by import xxx or from xxx import xxx.

Each time the pyc file is used, it will be compared according to the creation time of the pyc file and the source module. If the source module is modified, the pyc file will be recreated and the previous pyc file will be overwritten. If there is no modification, use the pyc file instead of the module directly.

The pyc file is stored in the _ _ pycache__ folder of the directory where the module is located.

As shown in the following figure, the corresponding pyc file will be generated when modula_a is imported by module_main, but module_main will not generate pyc file!

In addition, Python's pyc is not the same as JAVA's bytecode!

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