GitHub - hgrecco/flexparser
Why write another parser? I have asked myself the same question while working on this project. It is clear that there are excellent parsers out there, but I wanted to experiment with another way of writing them.
The idea is quite simple. You write a class for every type of content (called here ParsedStatement) you need to parse. Each class should have a from_string constructor. We used the typing module extensively to make the output structure easy to use and less error-prone.
For example:
fromdataclassesimportdataclassimportflexparserasfp@dataclass(frozen=True)classConfig: """Store here any configuration setting that the parser need (if any)."""@dataclass(frozen=True)classAssigment(fp.ParsedStatement[Config]): """Parses the following `this <- other`"""lhs: strrhs: str@classmethoddeffrom_string(cls, s): lhs, rhs=s.split("<-") returncls(lhs.strip(), rhs.strip())(Using a frozen dataclass is not necessary but is convenient. Being a dataclass, you get __init__, __str__, __repr__, etc. for free. Being frozen, sort of immutable, makes them easier to reason around.)
In certain cases, you might want to signal the parser that this class is not appropriate to parse the statement.
@dataclass(frozen=True)classAssigment(fp.ParsedStatement[Config]): """Parses the following `this <- other`"""lhs: strrhs: str@classmethoddeffrom_string(cls, s): if"<-"notins: returnNone# This means: I do not know how to parse itlhs, rhs=s.split("<-") returncls(lhs.strip(), rhs.strip())You might also want to indicate that this is the right ParsedStatement but something is not right:
@dataclass(frozen=True)classInvalidIdentifier(fp.ParsingError): value: str@dataclass(frozen=True)classAssigment(fp.ParsedStatement[Config]): """Parses the following `this <- other`"""lhs: strrhs: str@classmethoddeffrom_string(cls, s): if"<-"notins: returnNonelhs, rhs= (p.strip() forpins.split("<-")) ifnotstr.isidentifier(lhs): returnInvalidIdentifier(lhs) returncls(lhs, rhs)Put this into source.txt:
one <- other 2two <- new three <- newvalue one == three and then run the following code:
parsed=fp.parse("source.txt", Assigment) forelinparsed.iter_statements(): print(repr(el))will produce the following output:
BOF(start_line=0, ...) Assigment(start_line=1, ..., lhs='one', rhs='other') InvalidIdentifier(start_line=2, ..., value='2two') Assigment(start_line=3, ..., lhs='three', rhs='newvalue') UnknownStatement(start_line=4, ..., raw='one == three') EOS(start_line=5, ...) Now let's say we want to support equality comparison:
@dataclass(frozen=True)classEqualityComparison(fp.ParsedStatement[Config]): """Parses the following `this == other`"""lhs: strrhs: str@classmethoddeffrom_string(cls, s): if"=="notins: returnNonelhs, rhs= (p.strip() forpins.split("==")) returncls(lhs, rhs) parsed=fp.parse("source.txt", (Assigment, EqualityComparison)) forelinparsed.iter_statements(): print(repr(el))and run it again:
BOF(start_line=0, ...) Assigment(start_line=1, ..., lhs='one', rhs='other') InvalidIdentifier(start_line=2, ..., value='2two') Assigment(start_line=3, ..., lhs='three', rhs='newvalue') EqualityComparison(start_line=4, ..., lhs='one', rhs='three') EOS(start_line=5, ...) For multiple source files, flexparser provides the IncludeStatement base class:
@dataclass(frozen=True)classInclude(fp.IncludeStatement[Config]): """A naive implementation of #include "file" """value: str@classmethoddeffrom_string(cls, s): ifnots.startswith("#include "): returnNonevalue=s[len("#include ") :].strip().strip('"') returncls(value) @propertydeftarget(self): returnself.valueThis project was started as part of
, the Python units package. Take a look at its parser for a full example
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