def value(self):
"""
Return the parsed btye string as a DataFrame
"""
try:
return self._value
except AttributeError:
from io import BytesIO
from dask.bytes.utils import read_block
# read the relevant bytes
with open(self.filename, 'rb') as f:
block = read_block(f, self.offset, self.blocksize,
self.delimiter)
# parse the byte string
b = BytesIO()
b.write(block)
b.seek(0)
self._value = read_csv(b, **self.config)
return self._value
def test_read_block():
delimiter = b'\n'
data = delimiter.join([b'123', b'456', b'789'])
f = io.BytesIO(data)
assert read_block(f, 1, 2) == b'23'
assert read_block(f, 0, 1, delimiter=b'\n') == b'123\n'
assert read_block(f, 0, 2, delimiter=b'\n') == b'123\n'
assert read_block(f, 0, 3, delimiter=b'\n') == b'123\n'
assert read_block(f, 0, 5, delimiter=b'\n') == b'123\n456\n'
assert read_block(f, 0, 8, delimiter=b'\n') == b'123\n456\n789'
assert read_block(f, 0, 100, delimiter=b'\n') == b'123\n456\n789'
assert read_block(f, 1, 1, delimiter=b'\n') == b''
assert read_block(f, 1, 5, delimiter=b'\n') == b'456\n'
assert read_block(f, 1, 8, delimiter=b'\n') == b'456\n789'
for ols in [[(0, 3), (3, 3), (6, 3), (9, 2)],
[(0, 4), (4, 4), (8, 4)]]:
out = [read_block(f, o, l, b'\n') for o, l in ols]
assert b"".join(filter(None, out)) == data
def make_partitions(filename, blocksize, config, delimiter="\n"):
"""
Partition a CSV file into blocks, using the preferred blocksize
in bytes, returning the partititions and number of rows in
each partition
This divides the input file into partitions with size
roughly equal to blocksize, reads the bytes, and counts
the number of delimiters to compute the size of each block
Parameters
----------
filename : str
the name of the CSV file to load
blocksize : int
the desired number of bytes per block
delimiter : str, optional
the character separating lines; default is
the newline character
config : dict
any keyword options to pass to :func:`pandas.read_csv`
Returns
-------
partitions : list of CSVPartition
list of objects storing the data content of each file partition,
stored as a bytestring
sizes : list of int
the list of the number of rows in each partition
"""
from dask.bytes.utils import read_block
config = config.copy()
# search for lines separated by this character
delimiter = delimiter.encode()
# size in bytes and byte offsets of each partition
size = os.path.getsize(filename)
offsets = list(range(0, size, int(blocksize)))
# skip blank lines
skip_blank_lines = config.get('skip_blank_lines', True)
# number of rows to read
nrows = config.pop('nrows', None)
sizes = []
partitions = []
with open(filename, 'rb') as f:
for i, offset in enumerate(offsets):
# skiprows only valid for first block
if i > 0 and 'skiprows' in config:
config.pop('skiprows')
# set nrows for this block
config['nrows'] = nrows
block = read_block(f, offset, blocksize, delimiter)
partitions.append(
CSVPartition(filename, offset, blocksize, delimiter, **config))
# count delimiter to get size
size = block.count(delimiter)
# account for blank lines
if skip_blank_lines:
size -= block.count(delimiter + delimiter)
if i == 0 and block.startswith(delimiter):
size -= 1
# account for skiprows
skiprows = config.get('skiprows', 0)
size -= skiprows
# account for nrows
if nrows is not None and nrows > 0:
if nrows < size:
sizes.append(nrows)
break
else:
nrows -= size # update for next block
# manually increase size if at end of the file and no newline
if i == len(offsets) - 1 and not block.endswith(delimiter):
size += 1
sizes.append(size)
return partitions, sizes
def test_read_block():
delimiter = b"\n"
data = delimiter.join([b"123", b"456", b"789"])
f = io.BytesIO(data)
assert read_block(f, 1, 2) == b"23"
assert read_block(f, 0, 1, delimiter=b"\n") == b"123\n"
assert read_block(f, 0, 2, delimiter=b"\n") == b"123\n"
assert read_block(f, 0, 3, delimiter=b"\n") == b"123\n"
assert read_block(f, 0, 5, delimiter=b"\n") == b"123\n456\n"
assert read_block(f, 0, 8, delimiter=b"\n") == b"123\n456\n789"
assert read_block(f, 0, 100, delimiter=b"\n") == b"123\n456\n789"
assert read_block(f, 1, 1, delimiter=b"\n") == b""
assert read_block(f, 1, 5, delimiter=b"\n") == b"456\n"
assert read_block(f, 1, 8, delimiter=b"\n") == b"456\n789"
for ols in [[(0, 3), (3, 3), (6, 3), (9, 2)], [(0, 4), (4, 4), (8, 4)]]:
out = [read_block(f, o, l, b"\n") for o, l in ols]
assert b"".join(filter(None, out)) == data