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parser 522 PyParsing m3u parser 541
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m3u
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In this subsection we will develop a PLY parser for the m3u format And just as we did in the previous implementations, the parser will return its results in the form of a list of Song (collectionsnamedtuple()) objects, each of which holds a title, a duration in seconds, and a lename Since the format is so simple, PLY s lexer is suf cient to do all the parsing As before we will create a list of tokens, each one corresponding to a nonterminal in the BNF:
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tokens = ("M3U", "INFO", "SECONDS", "TITLE", "FILENAME")
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BNF 523
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We haven t got an ENTRY token this nonterminal is made up of a SECONDS and a TITLE Instead we de ne two states, called entry and filename When the lexer is in the entry state we will try to read the SECONDS and the TITLE, that is, an ENTRY, and when the lexer is in the filename state we will try to read the FILENAME To make PLY understand states we must create a states variable that is set to a list of one or more 2-tuples The rst item in each of the tuples is a state name and the second item is the state s type, either inclusive (ie, this state is in addition to the current state) or exclusive (ie, this state is the only active
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14 Introduction to Parsing
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state) PLY prede nes the INITIAL state which all lexers start in Here is the de nition of the states variable for the PLY m3u parser:
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states = (("entry", "exclusive"), ("filename", "exclusive"))
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Now that we have de ned our tokens and our states we can de ne the regexes and functions to match the BNF
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t_M3U = r"\#EXTM3U" def t_INFO(t): r"\#EXTINF:" tlexerbegin("entry") return None def t_entry_SECONDS(t): r"- \d+," tvalue = int(tvalue[:-1]) return t def t_entry_TITLE(t): r"[^\n]+" tlexerbegin("filename") return t def t_filename_FILENAME(t): r"[^\n]+" tlexerbegin("INITIAL") return t
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By default, the tokens, regexes, and functions operate in the INITIAL state However, we can specify that they are active in only one particular state by embedding the state s name after the t_ pre x So in this case the t_M3U regex and the t_INFO() function will match only in the INITIAL state, the t_entry_SECONDS() and t_entry_TITLE() functions will match only in the entry state, and the t_filename_FILENAME() function will match only in the filename state The lexer s state is changed by calling the lexer object s begin() method with the new state s name as its argument So in this example, when we match the INFO token we switch to the entry state; now only the SECONDS and TITLE tokens can match Once we have matched a TITLE we switch to the filename state, and once we have matched a FILENAME we switch back to the INITIAL state ready to match the next INFO token Notice that in the case of the t_INFO() function we return None; this means that the token will be discarded, which is correct since although we must match #EXTINF: for each entry, we don t need that text For the t_entry_SECONDS() function, we strip off the trailing comma and replace the token s value with the integer number of seconds
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Lex/Yacc-Style Parsing with PLY
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In this parser we want to ignore spurious whitespace that may occur between tokens, and we want to do so regardless of the state the lexer is in This can be achieved by creating a t_ignore variable, and by giving it a state of ANY which means it is active in any state:
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t_ANY_ignore = " \t\n"
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This will ensure that any whitespace between tokens is safely and conveniently ignored We have also de ned two functions, t_ANY_newline() and t_ANY_error(); these have exactly the same bodies as the t_newline() and t_error() functions de ned in the previous subsection (556 ) so neither are shown here but include the state of ANY in their names so that they are active no matter what state the lexer is in
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songs = [] title = seconds = None lexer = plylexlex() lexerinput(fhread()) for token in lexer: if tokentype == "SECONDS": seconds = tokenvalue elif tokentype == "TITLE": title = tokenvalue elif tokentype == "FILENAME": if title is not None and seconds is not None: songsappend(Song(title, seconds, tokenvalue)) title = seconds = None else: print("Failed, filename '{0}' without title/duration" format(tokenvalue))
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We use the lexer in the same way as we did for the pls lexer, iterating over the tokens, accumulating values (for the seconds and title), and whenever we get a lename to go with the seconds and title, adding a new song to the song list As before, at the end (not shown), we return the key_values dictionary to the caller