diff --git a/internals/parser.rst b/internals/parser.rst index ad97950ef8..688ad61e77 100644 --- a/internals/parser.rst +++ b/internals/parser.rst @@ -6,921 +6,5 @@ Guide to the parser .. highlight:: none -Abstract -======== - -The Parser in CPython is currently a `PEG (Parser Expression Grammar) -`_ parser. The first -version of the parser used to be an `LL(1) -`_ based parser that was one of the -oldest parts of CPython implemented before it was replaced by :pep:`617`. In -particular, both the current parser and the old LL(1) parser are the output of a -`parser generator `_. This -means that the way the parser is written is by feeding a description of the -Grammar of the Python language to a special program (the parser generator) which -outputs the parser. The way the Python language is changed is therefore by -modifying the grammar file and developers rarely need to interact with the -parser generator itself other than use it to generate the parser. - -How PEG parsers work -==================== - -.. _how-peg-parsers-work: - -A PEG (Parsing Expression Grammar) grammar (like the current one) differs from a -context-free grammar in that the way it is written more closely -reflects how the parser will operate when parsing it. The fundamental technical -difference is that the choice operator is ordered. This means that when writing:: - - rule: A | B | C - -a context-free-grammar parser (like an LL(1) parser) will generate constructions -that given an input string will *deduce* which alternative (``A``, ``B`` or ``C``) -must be expanded, while a PEG parser will check if the first alternative succeeds -and only if it fails, will it continue with the second or the third one in the -order in which they are written. This makes the choice operator not commutative. - -Unlike LL(1) parsers, PEG-based parsers cannot be ambiguous: if a string parses, -it has exactly one valid parse tree. This means that a PEG-based parser cannot -suffer from the ambiguity problems that can arise with LL(1) parsers and with -context-free grammars in general. - -PEG parsers are usually constructed as a recursive descent parser in which every -rule in the grammar corresponds to a function in the program implementing the -parser and the parsing expression (the "expansion" or "definition" of the rule) -represents the "code" in said function. Each parsing function conceptually takes -an input string as its argument, and yields one of the following results: - -* A "success" result. This result indicates that the expression can be parsed by - that rule and the function may optionally move forward or consume one or more - characters of the input string supplied to it. -* A "failure" result, in which case no input is consumed. - -Notice that "failure" results do not imply that the program is incorrect, nor do -they necessarily mean that the parsing has failed. Since the choice operator is -ordered, a failure very often merely indicates "try the following option". A -direct implementation of a PEG parser as a recursive descent parser will present -exponential time performance in the worst case, because PEG parsers have -infinite lookahead (this means that they can consider an arbitrary number of -tokens before deciding for a rule). Usually, PEG parsers avoid this exponential -time complexity with a technique called "packrat parsing" [1]_ which not only -loads the entire program in memory before parsing it but also allows the parser -to backtrack arbitrarily. This is made efficient by memoizing the rules already -matched for each position. The cost of the memoization cache is that the parser -will naturally use more memory than a simple LL(1) parser, which normally are -table-based. - - -Key ideas ---------- - -.. important:: - Don't try to reason about a PEG grammar in the same way you would to with an EBNF - or context free grammar. PEG is optimized to describe **how** input strings will - be parsed, while context-free grammars are optimized to generate strings of the - language they describe (in EBNF, to know if a given string is in the language, you need - to do work to find out as it is not immediately obvious from the grammar). - -* Alternatives are ordered ( ``A | B`` is not the same as ``B | A`` ). -* If a rule returns a failure, it doesn't mean that the parsing has failed, - it just means "try something else". -* By default PEG parsers run in exponential time, which can be optimized to linear by - using memoization. -* If parsing fails completely (no rule succeeds in parsing all the input text), the - PEG parser doesn't have a concept of "where the :exc:`SyntaxError` is". - - -.. _consequences-of-ordered-choice: - -Consequences of the ordered choice operator -------------------------------------------- - -Although PEG may look like EBNF, its meaning is quite different. The fact -that in PEG parsers alternatives are ordered (which is at the core of how PEG -parsers work) has deep consequences, other than removing ambiguity. - -If a rule has two alternatives and the first of them succeeds, the second one is -**not** attempted even if the caller rule fails to parse the rest of the input. -Thus the parser is said to be "eager". To illustrate this, consider -the following two rules (in these examples, a token is an individual character): :: - - first_rule: ( 'a' | 'aa' ) 'a' - second_rule: ('aa' | 'a' ) 'a' - -In a regular EBNF grammar, both rules specify the language ``{aa, aaa}`` but -in PEG, one of these two rules accepts the string ``aaa`` but not the string -``aa``. The other does the opposite -- it accepts the string ``aa`` -but not the string ``aaa``. The rule ``('a'|'aa')'a'`` does -not accept ``aaa`` because ``'a'|'aa'`` consumes the first ``a``, letting the -final ``a`` in the rule consume the second, and leaving out the third ``a``. -As the rule has succeeded, no attempt is ever made to go back and let -``'a'|'aa'`` try the second alternative. The expression ``('aa'|'a')'a'`` does -not accept ``aa`` because ``'aa'|'a'`` accepts all of ``aa``, leaving nothing -for the final ``a``. Again, the second alternative of ``'aa'|'a'`` is not -tried. - -.. caution:: - - The effects of ordered choice, such as the ones illustrated above, may be hidden by many levels of rules. - -For this reason, writing rules where an alternative is contained in the next one is in almost all cases a mistake, -for example: :: - - my_rule: - | 'if' expression 'then' block - | 'if' expression 'then' block 'else' block - -In this example, the second alternative will never be tried because the first one will -succeed first (even if the input string has an ``'else' block`` that follows). To correctly -write this rule you can simply alter the order: :: - - my_rule: - | 'if' expression 'then' block 'else' block - | 'if' expression 'then' block - -In this case, if the input string doesn't have an ``'else' block``, the first alternative -will fail and the second will be attempted without said part. - -Syntax -====== - -The grammar consists of a sequence of rules of the form: :: - - rule_name: expression - -Optionally, a type can be included right after the rule name, which -specifies the return type of the C or Python function corresponding to -the rule: :: - - rule_name[return_type]: expression - -If the return type is omitted, then a ``void *`` is returned in C and an -``Any`` in Python. - -Grammar expressions -------------------- - -``# comment`` -^^^^^^^^^^^^^ - -Python-style comments. - -``e1 e2`` -^^^^^^^^^ - -Match ``e1``, then match ``e2``. - -:: - - rule_name: first_rule second_rule - -``e1 | e2`` -^^^^^^^^^^^ - -Match ``e1`` or ``e2``. - -The first alternative can also appear on the line after the rule name -for formatting purposes. In that case, a \| must be used before the -first alternative, like so: - -:: - - rule_name[return_type]: - | first_alt - | second_alt - -``( e )`` -^^^^^^^^^ - -Match ``e``. - -:: - - rule_name: (e) - -A slightly more complex and useful example includes using the grouping -operator together with the repeat operators: - -:: - - rule_name: (e1 e2)* - -``[ e ] or e?`` -^^^^^^^^^^^^^^^ - -Optionally match ``e``. - -:: - - rule_name: [e] - -A more useful example includes defining that a trailing comma is -optional: - -:: - - rule_name: e (',' e)* [','] - -``e*`` -^^^^^^ - -Match zero or more occurrences of ``e``. - -:: - - rule_name: (e1 e2)* - -``e+`` -^^^^^^ - -Match one or more occurrences of ``e``. - -:: - - rule_name: (e1 e2)+ - -``s.e+`` -^^^^^^^^ - -Match one or more occurrences of ``e``, separated by ``s``. The generated parse -tree does not include the separator. This is otherwise identical to -``(e (s e)*)``. - -:: - - rule_name: ','.e+ - -``&e`` -^^^^^^ - -.. _peg-positive-lookahead: - -Succeed if ``e`` can be parsed, without consuming any input. - -``!e`` -^^^^^^ - -.. _peg-negative-lookahead: - -Fail if ``e`` can be parsed, without consuming any input. - -An example taken from the Python grammar specifies that a primary -consists of an atom, which is not followed by a ``.`` or a ``(`` or a -``[``: - -:: - - primary: atom !'.' !'(' !'[' - -``~`` -^^^^^ - -Commit to the current alternative, even if it fails to parse (this is called -the "cut"). - -:: - - rule_name: '(' ~ some_rule ')' | some_alt - -In this example, if a left parenthesis is parsed, then the other -alternative won’t be considered, even if some_rule or ``)`` fail to be -parsed. - -Left recursion --------------- - -PEG parsers normally do not support left recursion but CPython's parser -generator implements a technique similar to the one described in Medeiros et al. -[2]_ but using the memoization cache instead of static variables. This approach -is closer to the one described in Warth et al. [3]_. This allows us to write not -only simple left-recursive rules but also more complicated rules that involve -indirect left-recursion like:: - - rule1: rule2 | 'a' - rule2: rule3 | 'b' - rule3: rule1 | 'c' - -and "hidden left-recursion" like:: - - rule: 'optional'? rule '@' some_other_rule - -Variables in the grammar ------------------------- - -A sub-expression can be named by preceding it with an identifier and an -``=`` sign. The name can then be used in the action (see below), like this: :: - - rule_name[return_type]: '(' a=some_other_rule ')' { a } - -Grammar actions ---------------- - -.. _peg-grammar-actions: - -To avoid the intermediate steps that obscure the relationship between the -grammar and the AST generation the PEG parser allows directly generating AST -nodes for a rule via grammar actions. Grammar actions are language-specific -expressions that are evaluated when a grammar rule is successfully parsed. These -expressions can be written in Python or C depending on the desired output of the -parser generator. This means that if one would want to generate a parser in -Python and another in C, two grammar files should be written, each one with a -different set of actions, keeping everything else apart from said actions -identical in both files. As an example of a grammar with Python actions, the -piece of the parser generator that parses grammar files is bootstrapped from a -meta-grammar file with Python actions that generate the grammar tree as a result -of the parsing. - -In the specific case of the PEG grammar for Python, having actions allows -directly describing how the AST is composed in the grammar itself, making it -more clear and maintainable. This AST generation process is supported by the use -of some helper functions that factor out common AST object manipulations and -some other required operations that are not directly related to the grammar. - -To indicate these actions each alternative can be followed by the action code -inside curly-braces, which specifies the return value of the alternative:: - - rule_name[return_type]: - | first_alt1 first_alt2 { first_alt1 } - | second_alt1 second_alt2 { second_alt1 } - -If the action is omitted, a default action is generated: - -* If there's a single name in the rule, it gets returned. - -* If there is more than one name in the rule, a collection with all parsed - expressions gets returned (the type of the collection will be different - in C and Python). - -This default behaviour is primarily made for very simple situations and for -debugging purposes. - -.. warning:: - - It's important that the actions don't mutate any AST nodes that are passed - into them via variables referring to other rules. The reason for mutation - being not allowed is that the AST nodes are cached by memoization and could - potentially be reused in a different context, where the mutation would be - invalid. If an action needs to change an AST node, it should instead make a - new copy of the node and change that. - -The full meta-grammar for the grammars supported by the PEG generator is: - -:: - - start[Grammar]: grammar ENDMARKER { grammar } - - grammar[Grammar]: - | metas rules { Grammar(rules, metas) } - | rules { Grammar(rules, []) } - - metas[MetaList]: - | meta metas { [meta] + metas } - | meta { [meta] } - - meta[MetaTuple]: - | "@" NAME NEWLINE { (name.string, None) } - | "@" a=NAME b=NAME NEWLINE { (a.string, b.string) } - | "@" NAME STRING NEWLINE { (name.string, literal_eval(string.string)) } - - rules[RuleList]: - | rule rules { [rule] + rules } - | rule { [rule] } - - rule[Rule]: - | rulename ":" alts NEWLINE INDENT more_alts DEDENT { - Rule(rulename[0], rulename[1], Rhs(alts.alts + more_alts.alts)) } - | rulename ":" NEWLINE INDENT more_alts DEDENT { Rule(rulename[0], rulename[1], more_alts) } - | rulename ":" alts NEWLINE { Rule(rulename[0], rulename[1], alts) } - - rulename[RuleName]: - | NAME '[' type=NAME '*' ']' {(name.string, type.string+"*")} - | NAME '[' type=NAME ']' {(name.string, type.string)} - | NAME {(name.string, None)} - - alts[Rhs]: - | alt "|" alts { Rhs([alt] + alts.alts)} - | alt { Rhs([alt]) } - - more_alts[Rhs]: - | "|" alts NEWLINE more_alts { Rhs(alts.alts + more_alts.alts) } - | "|" alts NEWLINE { Rhs(alts.alts) } - - alt[Alt]: - | items '$' action { Alt(items + [NamedItem(None, NameLeaf('ENDMARKER'))], action=action) } - | items '$' { Alt(items + [NamedItem(None, NameLeaf('ENDMARKER'))], action=None) } - | items action { Alt(items, action=action) } - | items { Alt(items, action=None) } - - items[NamedItemList]: - | named_item items { [named_item] + items } - | named_item { [named_item] } - - named_item[NamedItem]: - | NAME '=' ~ item {NamedItem(name.string, item)} - | item {NamedItem(None, item)} - | it=lookahead {NamedItem(None, it)} - - lookahead[LookaheadOrCut]: - | '&' ~ atom {PositiveLookahead(atom)} - | '!' ~ atom {NegativeLookahead(atom)} - | '~' {Cut()} - - item[Item]: - | '[' ~ alts ']' {Opt(alts)} - | atom '?' {Opt(atom)} - | atom '*' {Repeat0(atom)} - | atom '+' {Repeat1(atom)} - | sep=atom '.' node=atom '+' {Gather(sep, node)} - | atom {atom} - - atom[Plain]: - | '(' ~ alts ')' {Group(alts)} - | NAME {NameLeaf(name.string) } - | STRING {StringLeaf(string.string)} - - # Mini-grammar for the actions - - action[str]: "{" ~ target_atoms "}" { target_atoms } - - target_atoms[str]: - | target_atom target_atoms { target_atom + " " + target_atoms } - | target_atom { target_atom } - - target_atom[str]: - | "{" ~ target_atoms "}" { "{" + target_atoms + "}" } - | NAME { name.string } - | NUMBER { number.string } - | STRING { string.string } - | "?" { "?" } - | ":" { ":" } - -As an illustrative example this simple grammar file allows directly -generating a full parser that can parse simple arithmetic expressions and that -returns a valid C-based Python AST: - -:: - - start[mod_ty]: a=expr_stmt* ENDMARKER { _PyAST_Module(a, NULL, p->arena) } - expr_stmt[stmt_ty]: a=expr NEWLINE { _PyAST_Expr(a, EXTRA) } - - expr[expr_ty]: - | l=expr '+' r=term { _PyAST_BinOp(l, Add, r, EXTRA) } - | l=expr '-' r=term { _PyAST_BinOp(l, Sub, r, EXTRA) } - | term - - term[expr_ty]: - | l=term '*' r=factor { _PyAST_BinOp(l, Mult, r, EXTRA) } - | l=term '/' r=factor { _PyAST_BinOp(l, Div, r, EXTRA) } - | factor - - factor[expr_ty]: - | '(' e=expr ')' { e } - | atom - - atom[expr_ty]: - | NAME - | NUMBER - -Here ``EXTRA`` is a macro that expands to ``start_lineno, start_col_offset, -end_lineno, end_col_offset, p->arena``, those being variables automatically -injected by the parser; ``p`` points to an object that holds on to all state -for the parser. - -A similar grammar written to target Python AST objects: - -:: - - start[ast.Module]: a=expr_stmt* ENDMARKER { ast.Module(body=a or [] } - expr_stmt: a=expr NEWLINE { ast.Expr(value=a, EXTRA) } - - expr: - | l=expr '+' r=term { ast.BinOp(left=l, op=ast.Add(), right=r, EXTRA) } - | l=expr '-' r=term { ast.BinOp(left=l, op=ast.Sub(), right=r, EXTRA) } - | term - - term: - | l=term '*' r=factor { ast.BinOp(left=l, op=ast.Mult(), right=r, EXTRA) } - | l=term '/' r=factor { ast.BinOp(left=l, op=ast.Div(), right=r, EXTRA) } - | factor - - factor: - | '(' e=expr ')' { e } - | atom - - atom: - | NAME - | NUMBER - - -Pegen -===== - -Pegen is the parser generator used in CPython to produce the final PEG parser used by the interpreter. It is the -program that can be used to read the python grammar located in :cpy-file:`Grammar/python.gram` and produce the final C -parser. It contains the following pieces: - -* A parser generator that can read a grammar file and produce a PEG parser written in Python or C that can parse - said grammar. The generator is located at :cpy-file:`Tools/peg_generator/pegen`. -* A PEG meta-grammar that automatically generates a Python parser that is used for the parser generator itself - (this means that there are no manually-written parsers). The meta-grammar is - located at :cpy-file:`Tools/peg_generator/pegen/metagrammar.gram`. -* A generated parser (using the parser generator) that can directly produce C and Python AST objects. - -The source code for Pegen lives at :cpy-file:`Tools/peg_generator/pegen` but normally all typical commands to interact -with the parser generator are executed from the main makefile. - -How to regenerate the parser ----------------------------- - -Once you have made the changes to the grammar files, to regenerate the ``C`` -parser (the one used by the interpreter) just execute: :: - - make regen-pegen - -using the ``Makefile`` in the main directory. If you are on Windows you can -use the Visual Studio project files to regenerate the parser or to execute: :: - - ./PCbuild/build.bat --regen - -The generated parser file is located at :cpy-file:`Parser/parser.c`. - -How to regenerate the meta-parser ---------------------------------- - -The meta-grammar (the grammar that describes the grammar for the grammar files -themselves) is located at :cpy-file:`Tools/peg_generator/pegen/metagrammar.gram`. -Although it is very unlikely that you will ever need to modify it, if you make any modifications -to this file (in order to implement new Pegen features) you will need to regenerate -the meta-parser (the parser that parses the grammar files). To do so just execute: :: - - make regen-pegen-metaparser - -If you are on Windows you can use the Visual Studio project files -to regenerate the parser or to execute: :: - - ./PCbuild/build.bat --regen - - -Grammatical elements and rules ------------------------------- - -Pegen has some special grammatical elements and rules: - -* Strings with single quotes (') (for example, ``'class'``) denote KEYWORDS. -* Strings with double quotes (") (for example, ``"match"``) denote SOFT KEYWORDS. -* Uppercase names (for example, ``NAME``) denote tokens in the :cpy-file:`Grammar/Tokens` file. -* Rule names starting with ``invalid_`` are used for specialized syntax errors. - - - These rules are NOT used in the first pass of the parser. - - Only if the first pass fails to parse, a second pass including the invalid - rules will be executed. - - If the parser fails in the second phase with a generic syntax error, the - location of the generic failure of the first pass will be used (this avoids - reporting incorrect locations due to the invalid rules). - - The order of the alternatives involving invalid rules matter - (like any rule in PEG). - -Tokenization ------------- - -It is common among PEG parser frameworks that the parser does both the parsing and the tokenization, -but this does not happen in Pegen. The reason is that the Python language needs a custom tokenizer -to handle things like indentation boundaries, some special keywords like ``ASYNC`` and ``AWAIT`` -(for compatibility purposes), backtracking errors (such as unclosed parenthesis), dealing with encoding, -interactive mode and much more. Some of these reasons are also there for historical purposes, and some -others are useful even today. - -The list of tokens (all uppercase names in the grammar) that you can use can be found in the :cpy-file:`Grammar/Tokens` -file. If you change this file to add new tokens, make sure to regenerate the files by executing: :: - - make regen-token - -If you are on Windows you can use the Visual Studio project files to regenerate the tokens or to execute: :: - - ./PCbuild/build.bat --regen - -How tokens are generated and the rules governing this are completely up to the tokenizer -(:cpy-file:`Parser/lexer/` and :cpy-file:`Parser/tokenizer/`); -the parser just receives tokens from it. - -Memoization ------------ - -As described previously, to avoid exponential time complexity in the parser, memoization is used. - -The C parser used by Python is highly optimized and memoization can be expensive both in memory and time. Although -the memory cost is obvious (the parser needs memory for storing previous results in the cache) the execution time -cost comes for continuously checking if the given rule has a cache hit or not. In many situations, just parsing it -again can be faster. Pegen **disables memoization by default** except for rules with the special marker ``memo`` after -the rule name (and type, if present): :: - - rule_name[typr] (memo): - ... - -By selectively turning on memoization for a handful of rules, the parser becomes faster and uses less memory. - -.. note:: - Left-recursive rules always use memoization, since the implementation of left-recursion depends on it. - -To know if a new rule needs memoization or not, benchmarking is required -(comparing execution times and memory usage of some considerably big files with -and without memoization). There is a very simple instrumentation API available -in the generated C parse code that allows to measure how much each rule uses -memoization (check the :cpy-file:`Parser/pegen.c` file for more information) but it -needs to be manually activated. - -Automatic variables -------------------- - -To make writing actions easier, Pegen injects some automatic variables in the namespace available -when writing actions. In the C parser, some of these automatic variable names are: - -* ``p``: The parser structure. -* ``EXTRA``: This is a macro that expands to ``(_start_lineno, _start_col_offset, _end_lineno, _end_col_offset, p->arena)``, - which is normally used to create AST nodes as almost all constructors need these attributes to be provided. All of the - location variables are taken from the location information of the current token. - -Hard and soft keywords ----------------------- - -.. note:: - In the grammar files, keywords are defined using **single quotes** (for example, ``'class'``) while soft - keywords are defined using **double quotes** (for example, ``"match"``). - -There are two kinds of keywords allowed in pegen grammars: *hard* and *soft* -keywords. The difference between hard and soft keywords is that hard keywords -are always reserved words, even in positions where they make no sense (for example, ``x = class + 1``), -while soft keywords only get a special meaning in context. Trying to use a hard -keyword as a variable will always fail: - -.. code-block:: - - >>> class = 3 - File "", line 1 - class = 3 - ^ - SyntaxError: invalid syntax - >>> foo(class=3) - File "", line 1 - foo(class=3) - ^^^^^ - SyntaxError: invalid syntax - -While soft keywords don't have this limitation if used in a context other the one where they -are defined as keywords: - -.. code-block:: python - - >>> match = 45 - >>> foo(match="Yeah!") - -The ``match`` and ``case`` keywords are soft keywords, so that they are recognized as -keywords at the beginning of a match statement or case block respectively, but are -allowed to be used in other places as variable or argument names. - -You can get a list of all keywords defined in the grammar from Python: - -.. code-block:: python - - >>> import keyword - >>> keyword.kwlist - ['False', 'None', 'True', 'and', 'as', 'assert', 'async', 'await', 'break', - 'class', 'continue', 'def', 'del', 'elif', 'else', 'except', 'finally', 'for', - 'from', 'global', 'if', 'import', 'in', 'is', 'lambda', 'nonlocal', 'not', 'or', - 'pass', 'raise', 'return', 'try', 'while', 'with', 'yield'] - -as well as soft keywords: - -.. code-block:: python - - >>> import keyword - >>> keyword.softkwlist - ['_', 'case', 'match'] - -.. caution:: - Soft keywords can be a bit challenging to manage as they can be accepted in - places you don't intend to, given how the order alternatives behave in PEG - parsers (see :ref:`consequences of ordered choice section - ` for some background on this). In general, - try to define them in places where there is not a lot of alternatives. - -Error handling --------------- - -When a pegen-generated parser detects that an exception is raised, it will -**automatically stop parsing**, no matter what the current state of the parser -is and it will unwind the stack and report the exception. This means that if a -:ref:`rule action ` raises an exception all parsing will -stop at that exact point. This is done to allow to correctly propagate any -exception set by calling Python's C API functions. This also includes :exc:`SyntaxError` -exceptions and this is the main mechanism the parser uses to report custom syntax -error messages. - -.. note:: - Tokenizer errors are normally reported by raising exceptions but some special - tokenizer errors such as unclosed parenthesis will be reported only after the - parser finishes without returning anything. - -How syntax errors are reported ------------------------------- - -As described previously in the :ref:`how PEG parsers work section -`, PEG parsers don't have a defined concept of where -errors happened in the grammar, because a rule failure doesn't imply a -parsing failure like in context free grammars. This means that some heuristic -has to be used to report generic errors unless something is explicitly declared -as an error in the grammar. - -To report generic syntax errors, pegen uses a common heuristic in PEG parsers: -the location of *generic* syntax errors is reported in the furthest token that -was attempted to be matched but failed. This is only done if parsing has failed -(the parser returns ``NULL`` in C or ``None`` in Python) but no exception has -been raised. - -.. caution:: - Positive and negative lookaheads will try to match a token so they will affect - the location of generic syntax errors. Use them carefully at boundaries - between rules. - -As the Python grammar was primordially written as an LL(1) grammar, this heuristic -has an extremely high success rate, but some PEG features can have small effects, -such as :ref:`positive lookaheads ` and -:ref:`negative lookaheads `. - -To generate more precise syntax errors, custom rules are used. This is a common practice -also in context free grammars: the parser will try to accept some construct that is known -to be incorrect just to report a specific syntax error for that construct. In pegen grammars, -these rules start with the ``invalid_`` prefix. This is because trying to match these rules -normally has a performance impact on parsing (and can also affect the 'correct' grammar itself -in some tricky cases, depending on the ordering of the rules) so the generated parser acts in -two phases: - -1. The first phase will try to parse the input stream without taking into account rules that - start with the ``invalid_`` prefix. If the parsing succeeds it will return the generated AST - and the second phase will not be attempted. - -2. If the first phase failed, a second parsing attempt is done including the rules that start - with an ``invalid_`` prefix. By design this attempt **cannot succeed** and is only executed - to give to the invalid rules a chance to detect specific situations where custom, more precise, - syntax errors can be raised. This also allows to trade a bit of performance for precision reporting - errors: given that we know that the input text is invalid, there is no need to be fast because - the interpreter is going to stop anyway. - -.. important:: - When defining invalid rules: - - * Make sure all custom invalid rules raise :exc:`SyntaxError` exceptions (or a subclass of it). - * Make sure **all** invalid rules start with the ``invalid_`` prefix to not - impact performance of parsing correct Python code. - * Make sure the parser doesn't behave differently for regular rules when you introduce invalid rules - (see the :ref:`how PEG parsers work section ` for more information). - -You can find a collection of macros to raise specialized syntax errors in the -:cpy-file:`Parser/pegen.h` header file. These macros allow also to report ranges for -the custom errors that will be highlighted in the tracebacks that will be -displayed when the error is reported. - -.. tip:: - A good way to test if an invalid rule will be triggered when you expect is to test if introducing - a syntax error **after** valid code triggers the rule or not. For example: :: - - $ 42 - - Should trigger the syntax error in the ``$`` character. If your rule is not correctly defined this - won't happen. For example, if you try to define a rule to match Python 2 style ``print`` statements - to make a better error message and you define it as: :: - - invalid_print: "print" expression - - This will **seem** to work because the parser will correctly parse ``print(something)`` because it is valid - code and the second phase will never execute but if you try to parse ``print(something) $ 3`` the first pass - of the parser will fail (because of the ``$``) and in the second phase, the rule will match the - ``print(something)`` as ``print`` followed by the variable ``something`` between parentheses and the error - will be reported there instead of the ``$`` character. - -Generating AST objects ----------------------- - -The output of the C parser used by CPython that is generated by the -:cpy-file:`Grammar/python.gram` grammar file is a Python AST object (using C -structures). This means that the actions in the grammar file generate AST objects -when they succeed. Constructing these objects can be quite cumbersome (see the -`AST compiler section `_ -for more information on how these objects are constructed and how they are used -by the compiler) so special helper functions are used. These functions are declared in the -:cpy-file:`Parser/pegen.h` header file and defined in the :cpy-file:`Parser/action_helpers.c` -file. These functions allow you to join AST sequences, get specific elements -from them or to do extra processing on the generated tree. - -.. caution:: - Actions must **never** be used to accept or reject rules. It may be tempting - in some situations to write a very generic rule and then check the generated - AST to decide if is valid or not but this will render the `official grammar - `_ partially incorrect - (because actions are not included) and will make it more difficult for other - Python implementations to adapt the grammar to their own needs. - -As a general rule, if an action spawns multiple lines or requires something more -complicated than a single expression of C code, is normally better to create a -custom helper in :cpy-file:`Parser/action_helpers.c` and expose it in the -:cpy-file:`Parser/pegen.h` header file so it can be used from the grammar. - -If the parsing succeeds, the parser **must** return a **valid** AST object. - -Testing -======= - -There are three files that contain tests for the grammar and the parser: - -* :cpy-file:`Lib/test/test_grammar.py` -* :cpy-file:`Lib/test/test_syntax.py` -* :cpy-file:`Lib/test/test_exceptions.py` - -Check the contents of these files to know which is the best place to place new tests depending -on the nature of the new feature you are adding. - -Tests for the parser generator itself can be found in the :cpy-file:`Lib/test/test_peg_generator` directory. - - -Debugging generated parsers -=========================== - -Making experiments ------------------- - -As the generated C parser is the one used by Python, this means that if something goes wrong when adding some -new rules to the grammar you cannot correctly compile and execute Python anymore. This makes it a bit challenging -to debug when something goes wrong, especially when making experiments. - -For this reason it is a good idea to experiment first by generating a Python parser. To do this, you can go to the -:cpy-file:`Tools/peg_generator/` directory on the CPython repository and manually call the parser generator by executing: - -.. code-block:: shell - - $ python -m pegen python - -This will generate a file called :file:`parse.py` in the same directory that you can use to parse some input: - -.. code-block:: shell - - $ python parse.py file_with_source_code_to_test.py - -As the generated :file:`parse.py` file is just Python code, you can modify it and add breakpoints to debug or -better understand some complex situations. - - -Verbose mode ------------- - -When Python is compiled in debug mode (by adding ``--with-pydebug`` when running the configure step in Linux or by -adding ``-d`` when calling the :cpy-file:`PCbuild/build.bat` script in Windows), it is possible to activate a **very** verbose -mode in the generated parser. This is very useful to debug the generated parser and to understand how it works, but it -can be a bit hard to understand at first. - -.. note:: - - When activating verbose mode in the Python parser, it is better to not use interactive mode as it can be much harder to - understand, because interactive mode involves some special steps compared to regular parsing. - -To activate verbose mode you can add the ``-d`` flag when executing Python: - -.. code-block:: shell - - $ python -d file_to_test.py - -This will print **a lot** of output to ``stderr`` so is probably better to dump it to a file for further analysis. The output -consists of trace lines with the following structure:: - - ('>'|'-'|'+'|'!') []: ... - -Every line is indented by a different amount (````) depending on how deep the call stack is. The next -character marks the type of the trace: - -* ``>`` indicates that a rule is going to be attempted to be parsed. -* ``-`` indicates that a rule has failed to be parsed. -* ``+`` indicates that a rule has been parsed correctly. -* ``!`` indicates that an exception or an error has been detected and the parser is unwinding. - -The ```` part indicates the current index in the token array, -the ```` part indicates what rule is being parsed and -the ```` part indicates what alternative within that rule -is being attempted. - - -References -========== - -.. [1] Ford, Bryan - https://pdos.csail.mit.edu/~baford/packrat/thesis/ - -.. [2] Medeiros et al. - https://arxiv.org/pdf/1207.0443 - -.. [3] Warth et al. - http://web.cs.ucla.edu/~todd/research/pepm08.pdf - - -.. admonition:: Document history - :class: note - - Pablo Galindo Salgado - Original author +This document is now part of the +`CPython Internals Docs `_.