The expr method itself uses the helper method eat to verify that the token type passed to the eat method matches the current token type. After matching the passed token type the eat method gets the next token and assigns it to the current_token variable, thus effectively eating the currently matched token and advancing the imaginary pointer in the stream of tokens. If the structure in the stream of tokens doesnt correspond to the expected integer plus integer sequence of tokens the eat method throws an exception. Lets recap what your interpreter does to evaluate an arithmetic expression: The interpreter accepts an input string, lets say 35 The interpreter calls the expr method to find a structure in the stream of tokens returned by the lexical analyzer get_next_token. The structure it tries to find is of the form integer plus integer. After its confirmed the structure, it interprets the input by adding the values of two integer tokens because its clear to the interpreter at that point that what it needs to do is add two integers, 3 and.
Python - the digital Cat
When you call the get_next_token method again the method checks if its a digit, which it is, so it increments pos and returns a new integer token with the value of the token set to integer 5 : Because the pos index is now past. Get_next_token token(integer, 3) interpreter. Get_next_token token(integer, 5) interpreter. Get_next_token token(eof, none) so now that your interpreter has access to the stream of tokens made from the input characters, the interpreter needs to do something with it: it needs to find the structure in the flat stream of tokens it gets from the lexer. Your interpreter expects to find the following structure in that stream: integer - allama plus - integer. That is, it tries to find a sequence of tokens: integer followed by a plus sign followed by an integer. The method responsible for finding and interpreting that structure is expr. This method verifies that the sequence of tokens does indeed correspond to the expected sequence of tokens,. E integer - plus - integer. After its successfully confirmed the structure, it generates the result by adding the value of the token on the left side of the plus and the right side of the plus, thus successfully interpreting the arithmetic getiton expression you passed to the interpreter.
They all mean the same: the part of your interpreter or compiler that turns the input of characters into a stream of tokens. The method get_next_token of the Interpreter class is presentation your lexical analyzer. Every time you call it, you get the next token created from the input of characters passed to the interpreter. Lets take a closer look at the method itself and see how it actually does its job of converting characters into tokens. The input is stored in the variable text that holds the input string and pos is an index into that string (think of the string as an array of characters). Pos is initially set to 0 and points to the character. The method first checks whether the character is a digit and if so, it increments pos and returns a token instance with the type integer and the value set to the integer value of the string 3, which is an integer 3 : The pos. The next time you call the method, it tests if a character at the position pos is a digit and then it tests if the character is a plus sign, which. As a result the method increments pos and returns a newly created token with the type plus and value : The pos now points to character.
Okay, now lets dive in and see how your interpreter works and how it dates evaluates arithmetic expressions. When you enter an expression 35 on the command line your interpreter gets a string. In order for the interpreter to actually understand what to do with that string it first book needs to break the input 35 into components called tokens. A token is an object that has a type and a value. For example, for the string 3 the type of the token will be integer and the corresponding value will be integer. The process of breaking the input string into tokens is called lexical analysis. So, the first step your interpreter needs to do is read the input of characters and convert it into a stream of tokens. The part of the interpreter that does it is called a lexical analyzer, or lexer for short. You might also encounter other names for the same component, like scanner or tokenizer.
Pos 1 return token ror def eat(self, token_type compare the current token type with the passed token type and if they match then "eat" the current token and assign the next token to the rrent_token, otherwise raise an exception. Type token_type: rrent_token t_next_token else: ror def expr(self "expr - integer plus integer" set current token to the first token taken from the input rrent_token t_next_token we expect the current token to be a single-digit integer left rrent_token self. Eat(integer) we expect the current token to be a token op rrent_token self. Eat(plus) we expect the current token to be a single-digit integer right rrent_token self. Eat(integer) after the above call the rrent_token is set to eof token at this point integer plus integer sequence of tokens has been successfully found and the method can just return the result of adding two integers, thus effectively. Expr print(result) if _name_ main main save the above code into file or download it directly from GitHub. Before you start digging deeper into the code, run the calculator on the command line and see it in action. Here is a sample session on my laptop (if you want to run the calculator under Python3 you will need to replace raw_input with input python calc 34 7 calc 35 8 calc 39 12 calc for your simple calculator to work properly without throwing. Dont worry, youll make it pretty complex pretty soon.
2) — the hitchhiker's guide
Examples: token(integer, 3) token(plus " return 'token(type, value. Type, valuerepr(lue) ) def _repr self return self._str class Interpreter (object def _init self, text client string input,. Text text self. Pos is an index into self. Pos 0 current token instance rrent_token none def error(self raise Exception Error parsing input def get_next_token(self "Lexical analyzer (also known as scanner or tokenizer) This method is responsible for breaking a sentence apart into tokens. One token at a time. Text is self.
Pos index past the and end of the self. if so, then return eof token because there is no more input left to convert into tokens if self. Pos len(text) - 1: return token(eof, none) get a character new at the position self. Pos and decide what token to create based on the single character current_char textself. Pos if the character is a digit then convert it to integer, create an integer token, increment self. Pos index to point to the next character after the digit, and return the integer token if current_digit token token(integer, int(current_char) self. Pos 1 return token if current_char token token(plus, current_char) self.
Visually it looks something like this: I hope that by now youre convinced that you really want to study and build an interpreter and a compiler. What can you expect from this series on interpreters? Here is the deal. You and i are going to create a simple interpreter for a large subset of, pascal language. At the end of this series you will have a working Pascal interpreter and a source-level debugger like. You might ask, why pascal?
For one thing, its not a made-up language that I came up with just for this series: its a real programming language that has many important language constructs. And some old, but useful, cs books use pascal programming language in their examples (I understand that thats not a particularly compelling reason to choose a language to build an interpreter for, but I thought it would be nice for a change to learn. The implementation language of the pascal interpreter will be python, but you can use any language you want because the ideas presented dont depend on any particular implementation language. Okay, lets get down to business. You will start your first foray into interpreters and compilers by writing a simple interpreter of arithmetic expressions, also known as a calculator. Today the goal is pretty minimalistic: to make your calculator handle the addition of two single digit integers like. Here is the source code for your calculator, sorry, interpreter : token types eof (end-of-file) token is used to indicate that there is no more input left for lexical analysis integer, plus, eof 'integer 'plus 'eof' class token(object def _init self. Type type token value: 0, 1,. 3, 4, 5, 6, 7, 8, 9, or None lue value def _str self "String representation of the class instance.
Python, interpreter, written in, python
Okay, but what are interpreters and compilers? The goal of an interpreter or a compiler is to translate a source program in some high-level language into some other form. Pretty vague, isnt it? Just bear with me, later in the series you will learn exactly what the source program is translated into. At this point you may also wonder what the difference is between an interpreter and a compiler. For the purpose of this series, lets agree that if a translator translates a source program into machine guaranteed language, it is a compiler. If a translator processes and executes the source program without translating it into machine language first, it is an interpreter.
You really want to know how computers work. Often interpreters and compilers look like magic. And you shouldnt be comfortable with that magic. You want to demystify the process of building an interpreter and a compiler, understand how they work, and get in essay control of things. You want to create your own programming language or domain specific language. If you create one, you will also need to create either an interpreter or a compiler for. Recently, there has been a resurgence of interest in new programming languages. And you can see a new programming language pop up almost every day: Elixir, go, rust just to name a few.
you will know how they work in the end. And you will become a confident happy camper too. At least I hope. Why would you study interpreters and compilers? I will give you three reasons. To write an interpreter or a compiler you have to have a lot of technical skills that you need to use together. Writing an interpreter or a compiler will help you improve those skills and become a better software developer. As well, the skills you will learn are useful in writing any software, not just interpreters or compilers.
Python ) that give the language its wide application range. For a detailed description of the whole. Python /c api, see the separate, python /c api reference manual. If you dont know resumes how compilers work, then you dont know how computers work. If youre not 100 sure whether you know how compilers work, then you dont know how they work. — steve Yegge, there you have. It doesnt really matter whether youre a newbie or a seasoned software developer: if you dont know how compilers and interpreters work, then you dont know how computers work. So, do you know how compilers and interpreters work?
Extending and Embedding the, python, interpreter —, python.6.6rc1
This document describes how to write modules in c or C to extend the. Python interpreter with new modules. Those modules can not only define new functions but also new object types and proposal their methods. The document also describes how to embed the, python interpreter in another application, for use as an extension language. Finally, it shows how to compile and link extension modules so that they can be loaded dynamically (at run time) into the interpreter, if the underlying operating system supports this feature. This document assumes basic knowledge about. For an informal introduction to the language, see, the, python. The, python, language reference gives a more formal definition of the language. The, python, standard Library documents the existing object types, functions and modules (both built-in and written.