Given the intrinsic complexity of parsing, I would strongly advise that you read or at least skim this entire document before jumping into a big development project with PLY.
If you are using Python 2, you have to use Python 2. Since PLY was primarily developed as an instructional tool, you will find it to be fairly picky about token and grammar rule specification.
In part, this added formality is meant to catch common programming mistakes made by novice users. However, advanced users will also find such features to be useful when building complicated grammars for real programming languages. It should also be noted that PLY does not provide much in the way of bells and whistles e.
History of Scintilla and SciTE Contributors Thanks to all the people that have contributed patches, bug reports and suggestions. Source code and. I've reïnstalled my ssh server, so I also need to reïnstall my Python packages. I did that, but I still get the error: ImportError: No module named cyprus4u.info I. Implementing a JIT Compiled Language with Haskell and LLVM. Adapted by Stephen Diehl (@smdiehl) This is an open source project hosted on Github. Corrections and. that’s all?¶ Well, why not write your own? Contributing to Pygments is easy and fun. Take a look at the docs on lexer development and contact details.
Nor would I consider it to be a parsing framework. The rest of this document assumes that you are somewhat familiar with parsing theory, syntax directed translation, and the use of compiler construction tools such as lex and yacc in other programming languages. If you are unfamiliar with these topics, you will probably want to consult an introductory text such as "Compilers: In fact, the O'Reilly book can be used as a reference for PLY as the concepts are virtually identical.
The two tools are meant to work together. The output of yacc. However, this is entirely up to the user. Like its Unix counterpart, yacc. Http://cyprus4u.info/repository/literature-review-on-bullying.php fact, almost everything that is possible in traditional yacc should be supported in PLY.
The primary difference between yacc. Instead, PLY relies on reflection introspection to build its lexers and parsers. This means that there are no extra source files nor is there a special compiler construction step e. Since the generation of the parsing tables is relatively expensive, PLY caches the results and saves them to a file. If no changes are detected in the input source, the tables are read from the cache. Otherwise, they are regenerated. For example, suppose you're writing a programming language and a user supplied the following input string: The next section shows how this is done using lex.
After that, repeated calls to token Write A Lexer In Python tokens. The http://cyprus4u.info/repository/pay-to-get-nursing-book-review.php code shows how this works: So, you can write the above loop as follows: This object has attributes tok.
The following code shows an example of accessing these attributes: This list is always required and is used to perform a variety of validation checks. The tokens list is also used by the yacc. In the example, the following code specified the token names: For simple tokens, the regular expression can be specified as strings such as this note: Python raw strings are used since they are the most convenient way to write regular expression strings: Please click for source some kind of action needs to be performed, a token rule can be specified as a function.
For example, this rule matches numbers and converts the string into a Python integer. The function always takes a single argument which is an instance of LexToken. This object has attributes of t. The action function can modify the contents of the LexToken object as appropriate. However, when it is done, the resulting token should be returned. If no value is returned by the action function, the token is simply discarded and the next token read.
Patterns are compiled using the re. However, be aware that unescaped whitespace is ignored and comments are allowed in this mode. If you need to match the character, use [ ]. All tokens defined by functions are added in the same order as they appear in the lexer file. Tokens defined by strings are added next by sorting them in order of decreasing regular expression length longer expressions are added Write A Lexer In Python. Without this ordering, it can be difficult to correctly match certain types of tokens.
By sorting regular expressions in order of decreasing length, this problem is solved for rules defined as strings. For functions, the order can be explicitly controlled since rules appearing first are checked first.
To handle reserved words, you should write a single rule to match an identifier and do a special name lookup in a function like this: You should avoid writing individual rules for reserved words.
This is probably not what you want. Normally, the value is the text that was matched. However, the value can be assigned to any Python object. For instance, when lexing identifiers, you may want to return both the identifier name and information from some sort of symbol table.
To do this, you might write a rule like this: Look up symbol table information and return a tuple t. Thus, accessing other attributes may be unnecessarily awkward. If you need to store multiple values Write A Lexer In Python a token, assign a tuple, dictionary, or instance to value. This is because lex. To update this information, you need to write a special rule. After the line number is updated, the token is simply discarded since nothing is returned.
However, it does record positional information related to each token in the lexpos attribute. Using this, it is usually possible to compute column information as a separate step.
For instance, just count backwards until you reach a newline. Usually this is used to skip over whitespace and other non-essential characters.
(1) Writing a programming language - the Lexer
For example, if you had a rule to capture quoted text, that pattern can include the ignored characters which will be captured in the normal way. Literal characters can be specified by defining a variable literals in your lexing module. Literals are checked after all of the defined regular expression rules. Thus, if a rule starts with one of the literal characters, it will always take precedence. When a literal token is returned, both its type and value attributes are set to the character itself.
It's possible to write token functions that perform additional actions when literals are matched. However, you'll need to set the token type appropriately. In this case, the t. In the example, the error function was defined as follows: As input, it receives a token type 'eof' with the lineno and lexpos attributes set appropriately. The main use of this function is provide more input to the lexer so that it can continue to parse.
Here is an example of how this works:. The EOF function should return the next available token by calling self. Be aware that setting more input with the self. The lexpos attribute is reset so be aware of that if you're using it in error reporting. This function uses Python reflection or introspection to read the regular expression rules out of the calling context and build the lexer.
Once the lexer has been built, two methods can be used to control the lexer. To change the name of the lexer-generated module, use the lextab keyword argument. This will produce various read more of debugging information including all of the added rules, the master regular expressions used by the lexer, and tokens generating during lexing.
To use it, simply put this in your lexer:. For example, you might have a dedicated module that just contains the token rules: List of token names. This is because PLY only works properly if the lexer actions are defined by bound-methods.
When using the module option to lexPLY collects symbols from the underlying object using the dir function. Finally, if you want to keep things nicely encapsulated, but don't want to use a full-fledged class definition, lexers can be defined using closures. If you are defining a lexer using a class or closure, be aware that PLY still requires you to only define a single lexer per module source file.
One way to do this is to keep a set of global variables in the module where you created the lexer. To this, you can use the lexer attribute of tokens passed to the various rules. However, this might also feel like a gross violation of encapsulation to OO purists.
Just to put your mind at some ease, all internal attributes of the lexer with the exception of lineno have names that are prefixed by lex e. Thus, it is perfectly safe to store attributes in the lexer that don't have names starting with that prefix or please click for source name that conflicts with one of the predefined methods e.
If you don't like assigning values on the lexer object, you can define your lexer as a class as shown in the previous section: State can Write A Lexer In Python be managed through closures.
For example, in Python 3: