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514 lines
19 KiB
Python
514 lines
19 KiB
Python
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r"""
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CSV parsing and writing.
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This module provides classes that assist in the reading and writing
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of Comma Separated Value (CSV) files, and implements the interface
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described by PEP 305. Although many CSV files are simple to parse,
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the format is not formally defined by a stable specification and
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is subtle enough that parsing lines of a CSV file with something
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like line.split(",") is bound to fail. The module supports three
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basic APIs: reading, writing, and registration of dialects.
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DIALECT REGISTRATION:
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Readers and writers support a dialect argument, which is a convenient
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handle on a group of settings. When the dialect argument is a string,
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it identifies one of the dialects previously registered with the module.
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If it is a class or instance, the attributes of the argument are used as
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the settings for the reader or writer:
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class excel:
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delimiter = ','
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quotechar = '"'
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escapechar = None
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doublequote = True
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skipinitialspace = False
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lineterminator = '\r\n'
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quoting = QUOTE_MINIMAL
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SETTINGS:
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* quotechar - specifies a one-character string to use as the
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quoting character. It defaults to '"'.
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* delimiter - specifies a one-character string to use as the
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field separator. It defaults to ','.
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* skipinitialspace - specifies how to interpret spaces which
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immediately follow a delimiter. It defaults to False, which
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means that spaces immediately following a delimiter is part
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of the following field.
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* lineterminator - specifies the character sequence which should
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terminate rows.
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* quoting - controls when quotes should be generated by the writer.
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It can take on any of the following module constants:
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csv.QUOTE_MINIMAL means only when required, for example, when a
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field contains either the quotechar or the delimiter
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csv.QUOTE_ALL means that quotes are always placed around fields.
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csv.QUOTE_NONNUMERIC means that quotes are always placed around
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fields which do not parse as integers or floating-point
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numbers.
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csv.QUOTE_STRINGS means that quotes are always placed around
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fields which are strings. Note that the Python value None
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is not a string.
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csv.QUOTE_NOTNULL means that quotes are only placed around fields
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that are not the Python value None.
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csv.QUOTE_NONE means that quotes are never placed around fields.
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* escapechar - specifies a one-character string used to escape
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the delimiter when quoting is set to QUOTE_NONE.
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* doublequote - controls the handling of quotes inside fields. When
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True, two consecutive quotes are interpreted as one during read,
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and when writing, each quote character embedded in the data is
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written as two quotes
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"""
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import re
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import types
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from _csv import Error, writer, reader, register_dialect, \
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unregister_dialect, get_dialect, list_dialects, \
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field_size_limit, \
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QUOTE_MINIMAL, QUOTE_ALL, QUOTE_NONNUMERIC, QUOTE_NONE, \
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QUOTE_STRINGS, QUOTE_NOTNULL
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from _csv import Dialect as _Dialect
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from io import StringIO
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__all__ = ["QUOTE_MINIMAL", "QUOTE_ALL", "QUOTE_NONNUMERIC", "QUOTE_NONE",
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"QUOTE_STRINGS", "QUOTE_NOTNULL",
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"Error", "Dialect", "excel", "excel_tab",
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"field_size_limit", "reader", "writer",
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"register_dialect", "get_dialect", "list_dialects", "Sniffer",
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"unregister_dialect", "DictReader", "DictWriter",
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"unix_dialect"]
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__version__ = "1.0"
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class Dialect:
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"""Describe a CSV dialect.
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This must be subclassed (see csv.excel). Valid attributes are:
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delimiter, quotechar, escapechar, doublequote, skipinitialspace,
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lineterminator, quoting.
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"""
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_name = ""
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_valid = False
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# placeholders
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delimiter = None
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quotechar = None
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escapechar = None
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doublequote = None
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skipinitialspace = None
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lineterminator = None
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quoting = None
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def __init__(self):
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if self.__class__ != Dialect:
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self._valid = True
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self._validate()
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def _validate(self):
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try:
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_Dialect(self)
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except TypeError as e:
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# Re-raise to get a traceback showing more user code.
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raise Error(str(e)) from None
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class excel(Dialect):
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"""Describe the usual properties of Excel-generated CSV files."""
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delimiter = ','
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quotechar = '"'
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doublequote = True
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skipinitialspace = False
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lineterminator = '\r\n'
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quoting = QUOTE_MINIMAL
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register_dialect("excel", excel)
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class excel_tab(excel):
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"""Describe the usual properties of Excel-generated TAB-delimited files."""
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delimiter = '\t'
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register_dialect("excel-tab", excel_tab)
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class unix_dialect(Dialect):
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"""Describe the usual properties of Unix-generated CSV files."""
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delimiter = ','
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quotechar = '"'
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doublequote = True
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skipinitialspace = False
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lineterminator = '\n'
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quoting = QUOTE_ALL
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register_dialect("unix", unix_dialect)
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class DictReader:
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def __init__(self, f, fieldnames=None, restkey=None, restval=None,
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dialect="excel", *args, **kwds):
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if fieldnames is not None and iter(fieldnames) is fieldnames:
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fieldnames = list(fieldnames)
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self._fieldnames = fieldnames # list of keys for the dict
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self.restkey = restkey # key to catch long rows
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self.restval = restval # default value for short rows
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self.reader = reader(f, dialect, *args, **kwds)
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self.dialect = dialect
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self.line_num = 0
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def __iter__(self):
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return self
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@property
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def fieldnames(self):
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if self._fieldnames is None:
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try:
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self._fieldnames = next(self.reader)
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except StopIteration:
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pass
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self.line_num = self.reader.line_num
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return self._fieldnames
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@fieldnames.setter
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def fieldnames(self, value):
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self._fieldnames = value
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def __next__(self):
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if self.line_num == 0:
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# Used only for its side effect.
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self.fieldnames
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row = next(self.reader)
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self.line_num = self.reader.line_num
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# unlike the basic reader, we prefer not to return blanks,
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# because we will typically wind up with a dict full of None
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# values
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while row == []:
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row = next(self.reader)
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d = dict(zip(self.fieldnames, row))
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lf = len(self.fieldnames)
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lr = len(row)
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if lf < lr:
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d[self.restkey] = row[lf:]
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elif lf > lr:
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for key in self.fieldnames[lr:]:
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d[key] = self.restval
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return d
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__class_getitem__ = classmethod(types.GenericAlias)
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class DictWriter:
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def __init__(self, f, fieldnames, restval="", extrasaction="raise",
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dialect="excel", *args, **kwds):
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if fieldnames is not None and iter(fieldnames) is fieldnames:
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fieldnames = list(fieldnames)
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self.fieldnames = fieldnames # list of keys for the dict
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self.restval = restval # for writing short dicts
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extrasaction = extrasaction.lower()
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if extrasaction not in ("raise", "ignore"):
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raise ValueError("extrasaction (%s) must be 'raise' or 'ignore'"
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% extrasaction)
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self.extrasaction = extrasaction
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self.writer = writer(f, dialect, *args, **kwds)
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def writeheader(self):
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header = dict(zip(self.fieldnames, self.fieldnames))
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return self.writerow(header)
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def _dict_to_list(self, rowdict):
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if self.extrasaction == "raise":
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wrong_fields = rowdict.keys() - self.fieldnames
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if wrong_fields:
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raise ValueError("dict contains fields not in fieldnames: "
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+ ", ".join([repr(x) for x in wrong_fields]))
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return (rowdict.get(key, self.restval) for key in self.fieldnames)
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def writerow(self, rowdict):
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return self.writer.writerow(self._dict_to_list(rowdict))
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def writerows(self, rowdicts):
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return self.writer.writerows(map(self._dict_to_list, rowdicts))
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__class_getitem__ = classmethod(types.GenericAlias)
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class Sniffer:
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'''
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"Sniffs" the format of a CSV file (i.e. delimiter, quotechar)
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Returns a Dialect object.
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'''
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def __init__(self):
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# in case there is more than one possible delimiter
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self.preferred = [',', '\t', ';', ' ', ':']
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def sniff(self, sample, delimiters=None):
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"""
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Returns a dialect (or None) corresponding to the sample
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"""
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quotechar, doublequote, delimiter, skipinitialspace = \
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self._guess_quote_and_delimiter(sample, delimiters)
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if not delimiter:
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delimiter, skipinitialspace = self._guess_delimiter(sample,
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delimiters)
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if not delimiter:
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raise Error("Could not determine delimiter")
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class dialect(Dialect):
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_name = "sniffed"
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lineterminator = '\r\n'
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quoting = QUOTE_MINIMAL
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# escapechar = ''
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dialect.doublequote = doublequote
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dialect.delimiter = delimiter
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# _csv.reader won't accept a quotechar of ''
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dialect.quotechar = quotechar or '"'
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dialect.skipinitialspace = skipinitialspace
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return dialect
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def _guess_quote_and_delimiter(self, data, delimiters):
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"""
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Looks for text enclosed between two identical quotes
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(the probable quotechar) which are preceded and followed
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by the same character (the probable delimiter).
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For example:
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,'some text',
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The quote with the most wins, same with the delimiter.
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If there is no quotechar the delimiter can't be determined
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this way.
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"""
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matches = []
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for restr in (r'(?P<delim>[^\w\n"\'])(?P<space> ?)(?P<quote>["\']).*?(?P=quote)(?P=delim)', # ,".*?",
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r'(?:^|\n)(?P<quote>["\']).*?(?P=quote)(?P<delim>[^\w\n"\'])(?P<space> ?)', # ".*?",
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r'(?P<delim>[^\w\n"\'])(?P<space> ?)(?P<quote>["\']).*?(?P=quote)(?:$|\n)', # ,".*?"
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r'(?:^|\n)(?P<quote>["\']).*?(?P=quote)(?:$|\n)'): # ".*?" (no delim, no space)
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regexp = re.compile(restr, re.DOTALL | re.MULTILINE)
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matches = regexp.findall(data)
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if matches:
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break
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if not matches:
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# (quotechar, doublequote, delimiter, skipinitialspace)
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return ('', False, None, 0)
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quotes = {}
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delims = {}
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spaces = 0
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groupindex = regexp.groupindex
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for m in matches:
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n = groupindex['quote'] - 1
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key = m[n]
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if key:
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quotes[key] = quotes.get(key, 0) + 1
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try:
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n = groupindex['delim'] - 1
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key = m[n]
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except KeyError:
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continue
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if key and (delimiters is None or key in delimiters):
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delims[key] = delims.get(key, 0) + 1
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try:
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n = groupindex['space'] - 1
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except KeyError:
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continue
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if m[n]:
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spaces += 1
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quotechar = max(quotes, key=quotes.get)
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if delims:
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delim = max(delims, key=delims.get)
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skipinitialspace = delims[delim] == spaces
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if delim == '\n': # most likely a file with a single column
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delim = ''
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else:
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# there is *no* delimiter, it's a single column of quoted data
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delim = ''
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skipinitialspace = 0
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# if we see an extra quote between delimiters, we've got a
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# double quoted format
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dq_regexp = re.compile(
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r"((%(delim)s)|^)\W*%(quote)s[^%(delim)s\n]*%(quote)s[^%(delim)s\n]*%(quote)s\W*((%(delim)s)|$)" % \
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{'delim':re.escape(delim), 'quote':quotechar}, re.MULTILINE)
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if dq_regexp.search(data):
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doublequote = True
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else:
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doublequote = False
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return (quotechar, doublequote, delim, skipinitialspace)
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def _guess_delimiter(self, data, delimiters):
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"""
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The delimiter /should/ occur the same number of times on
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each row. However, due to malformed data, it may not. We don't want
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an all or nothing approach, so we allow for small variations in this
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number.
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1) build a table of the frequency of each character on every line.
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2) build a table of frequencies of this frequency (meta-frequency?),
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e.g. 'x occurred 5 times in 10 rows, 6 times in 1000 rows,
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7 times in 2 rows'
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3) use the mode of the meta-frequency to determine the /expected/
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frequency for that character
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4) find out how often the character actually meets that goal
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5) the character that best meets its goal is the delimiter
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For performance reasons, the data is evaluated in chunks, so it can
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try and evaluate the smallest portion of the data possible, evaluating
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additional chunks as necessary.
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"""
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data = list(filter(None, data.split('\n')))
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ascii = [chr(c) for c in range(127)] # 7-bit ASCII
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# build frequency tables
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chunkLength = min(10, len(data))
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iteration = 0
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charFrequency = {}
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modes = {}
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delims = {}
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start, end = 0, chunkLength
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while start < len(data):
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iteration += 1
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for line in data[start:end]:
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for char in ascii:
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metaFrequency = charFrequency.get(char, {})
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# must count even if frequency is 0
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freq = line.count(char)
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# value is the mode
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metaFrequency[freq] = metaFrequency.get(freq, 0) + 1
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charFrequency[char] = metaFrequency
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for char in charFrequency.keys():
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items = list(charFrequency[char].items())
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if len(items) == 1 and items[0][0] == 0:
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continue
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# get the mode of the frequencies
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if len(items) > 1:
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modes[char] = max(items, key=lambda x: x[1])
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# adjust the mode - subtract the sum of all
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# other frequencies
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items.remove(modes[char])
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modes[char] = (modes[char][0], modes[char][1]
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- sum(item[1] for item in items))
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else:
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modes[char] = items[0]
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# build a list of possible delimiters
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modeList = modes.items()
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total = float(min(chunkLength * iteration, len(data)))
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# (rows of consistent data) / (number of rows) = 100%
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consistency = 1.0
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# minimum consistency threshold
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threshold = 0.9
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while len(delims) == 0 and consistency >= threshold:
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for k, v in modeList:
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if v[0] > 0 and v[1] > 0:
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if ((v[1]/total) >= consistency and
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(delimiters is None or k in delimiters)):
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delims[k] = v
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consistency -= 0.01
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if len(delims) == 1:
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delim = list(delims.keys())[0]
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skipinitialspace = (data[0].count(delim) ==
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data[0].count("%c " % delim))
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return (delim, skipinitialspace)
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# analyze another chunkLength lines
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start = end
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end += chunkLength
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if not delims:
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return ('', 0)
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# if there's more than one, fall back to a 'preferred' list
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if len(delims) > 1:
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for d in self.preferred:
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if d in delims.keys():
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skipinitialspace = (data[0].count(d) ==
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data[0].count("%c " % d))
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return (d, skipinitialspace)
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# nothing else indicates a preference, pick the character that
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# dominates(?)
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items = [(v,k) for (k,v) in delims.items()]
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items.sort()
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delim = items[-1][1]
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skipinitialspace = (data[0].count(delim) ==
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data[0].count("%c " % delim))
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return (delim, skipinitialspace)
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def has_header(self, sample):
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# Creates a dictionary of types of data in each column. If any
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# column is of a single type (say, integers), *except* for the first
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# row, then the first row is presumed to be labels. If the type
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# can't be determined, it is assumed to be a string in which case
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# the length of the string is the determining factor: if all of the
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# rows except for the first are the same length, it's a header.
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# Finally, a 'vote' is taken at the end for each column, adding or
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# subtracting from the likelihood of the first row being a header.
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rdr = reader(StringIO(sample), self.sniff(sample))
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header = next(rdr) # assume first row is header
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columns = len(header)
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columnTypes = {}
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for i in range(columns): columnTypes[i] = None
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checked = 0
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for row in rdr:
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# arbitrary number of rows to check, to keep it sane
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if checked > 20:
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break
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checked += 1
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if len(row) != columns:
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continue # skip rows that have irregular number of columns
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for col in list(columnTypes.keys()):
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thisType = complex
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try:
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thisType(row[col])
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except (ValueError, OverflowError):
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# fallback to length of string
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thisType = len(row[col])
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if thisType != columnTypes[col]:
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if columnTypes[col] is None: # add new column type
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columnTypes[col] = thisType
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else:
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# type is inconsistent, remove column from
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# consideration
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del columnTypes[col]
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# finally, compare results against first row and "vote"
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# on whether it's a header
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hasHeader = 0
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for col, colType in columnTypes.items():
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if isinstance(colType, int): # it's a length
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if len(header[col]) != colType:
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hasHeader += 1
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else:
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hasHeader -= 1
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else: # attempt typecast
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try:
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colType(header[col])
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except (ValueError, TypeError):
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hasHeader += 1
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else:
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hasHeader -= 1
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return hasHeader > 0
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