我知道如何读取字节 - x.read(number_of_bytes)
,但我怎样才能读取Python中的位?
我必须从二进制文件
中只读取5位(不是8位[1字节])任何想法或方法?
答案 0 :(得分:29)
Python一次只能读取一个字节。您需要读取一个完整的字节,然后只需从该字节中提取所需的值,例如
b = x.read(1)
firstfivebits = b >> 3
或者如果你想要5个最低有效位,而不是5个最高有效位:
b = x.read(1)
lastfivebits = b & 0b11111
可在此处找到其他一些有用的位操作信息:http://wiki.python.org/moin/BitManipulation
答案 1 :(得分:4)
正如公认的答案所述,标准Python I / O一次只能读写整个字节。但是,您可以使用Bitwise I/O的此配方模拟这样的位流。
<强>更新强>
修改Rosetta Code的Python版本后,在Python 2&amp; 3,我将这些变化纳入了这个答案。
除此之外,后来,在受到@mhernandez发表的评论的启发后,我进一步修改了Rosetta代码,因此它支持所谓的context manager protocol,它允许使用它的两个类的实例在Python with
语句中。最新版本如下所示:
class BitWriter(object):
def __init__(self, f):
self.accumulator = 0
self.bcount = 0
self.out = f
def __enter__(self):
return self
def __exit__(self, exc_type, exc_val, exc_tb):
self.flush()
def __del__(self):
try:
self.flush()
except ValueError: # I/O operation on closed file.
pass
def _writebit(self, bit):
if self.bcount == 8:
self.flush()
if bit > 0:
self.accumulator |= 1 << 7-self.bcount
self.bcount += 1
def writebits(self, bits, n):
while n > 0:
self._writebit(bits & 1 << n-1)
n -= 1
def flush(self):
self.out.write(bytearray([self.accumulator]))
self.accumulator = 0
self.bcount = 0
class BitReader(object):
def __init__(self, f):
self.input = f
self.accumulator = 0
self.bcount = 0
self.read = 0
def __enter__(self):
return self
def __exit__(self, exc_type, exc_val, exc_tb):
pass
def _readbit(self):
if not self.bcount:
a = self.input.read(1)
if a:
self.accumulator = ord(a)
self.bcount = 8
self.read = len(a)
rv = (self.accumulator & (1 << self.bcount-1)) >> self.bcount-1
self.bcount -= 1
return rv
def readbits(self, n):
v = 0
while n > 0:
v = (v << 1) | self._readbit()
n -= 1
return v
if __name__ == '__main__':
import os
import sys
# Determine this module's name from it's file name and import it.
module_name = os.path.splitext(os.path.basename(__file__))[0]
bitio = __import__(module_name)
with open('bitio_test.dat', 'wb') as outfile:
with bitio.BitWriter(outfile) as writer:
chars = '12345abcde'
for ch in chars:
writer.writebits(ord(ch), 7)
with open('bitio_test.dat', 'rb') as infile:
with bitio.BitReader(infile) as reader:
chars = []
while True:
x = reader.readbits(7)
if not reader.read: # End-of-file?
break
chars.append(chr(x))
print(''.join(chars))
另一个用法示例,说明如何“压缩”8位字节ASCII流,丢弃最重要的“未使用”位...并将其读回(但不要将其用作上下文管理器)。
import sys
import bitio
o = bitio.BitWriter(sys.stdout)
c = sys.stdin.read(1)
while len(c) > 0:
o.writebits(ord(c), 7)
c = sys.stdin.read(1)
o.flush()
...并“解散”相同的流:
import sys
import bitio
r = bitio.BitReader(sys.stdin)
while True:
x = r.readbits(7)
if not r.read: # nothing read
break
sys.stdout.write(chr(x))
答案 2 :(得分:0)
它出现在Google搜索的顶部,用于使用python读取位。
我发现bitstring
是一个读取位的好软件包,也是对本机功能的改进(对Python 3.6来说还不错),例如
# import module
from bitstring import ConstBitStream
# read file
b = ConstBitStream(filename='file.bin')
# read 5 bits
output = b.read(5)
# convert to unsigned int
integer_value = output.uint
更多文档和详细信息在这里: https://pythonhosted.org/bitstring/index.html