如何从Python中的文件一次读取单个字符？

Question

谁能告诉我怎么能这样做？

Answer 1

with open(filename) as f:
  while True:
    c = f.read(1)
    if not c:
      print "End of file"
      break
    print "Read a character:", c

Answer 2

首先打开一个文件:

with open("filename") as fileobj:
    for line in fileobj:  
       for ch in line: 
           print ch

Answer 3

我喜欢接受的答案:它很简单,可以完成工作.我还想提供一个替代实现:

def chunks(filename, buffer_size=4096):
    """Reads `filename` in chunks of `buffer_size` bytes and yields each chunk
    until no more characters can be read; the last chunk will most likely have
    less than `buffer_size` bytes.

    :param str filename: Path to the file
    :param int buffer_size: Buffer size, in bytes (default is 4096)
    :return: Yields chunks of `buffer_size` size until exhausting the file
    :rtype: str

    """
    with open(filename, "rb") as fp:
        chunk = fp.read(buffer_size)
        while chunk:
            yield chunk
            chunk = fp.read(buffer_size)

def chars(filename, buffersize=4096):
    """Yields the contents of file `filename` character-by-character. Warning:
    will only work for encodings where one character is encoded as one byte.

    :param str filename: Path to the file
    :param int buffer_size: Buffer size for the underlying chunks,
    in bytes (default is 4096)
    :return: Yields the contents of `filename` character-by-character.
    :rtype: char

    """
    for chunk in chunks(filename, buffersize):
        for char in chunk:
            yield char

def main(buffersize, filenames):
    """Reads several files character by character and redirects their contents
    to `/dev/null`.

    """
    for filename in filenames:
        with open("/dev/null", "wb") as fp:
            for char in chars(filename, buffersize):
                fp.write(char)

if __name__ == "__main__":
    # Try reading several files varying the buffer size
    import sys
    buffersize = int(sys.argv[1])
    filenames  = sys.argv[2:]
    sys.exit(main(buffersize, filenames))

我建议的代码与您接受的答案基本相同:从文件中读取给定的字节数.不同之处在于它首先读取了大量数据(4006是X86的一个很好的默认值,但你可能想尝试1024或8192;你的页面大小的任何倍数),然后它产生那个块中的字符一个一个人.

对于较大的文件,我提供的代码可能更快.以托尔斯泰为例,以战争与和平的全文为例.这些是我的计时结果(使用OS X 10.7.4的Mac Book Pro; so.py是我给我粘贴的代码的名称):

$ time python so.py 1 2600.txt.utf-8
python so.py 1 2600.txt.utf-8  3.79s user 0.01s system 99% cpu 3.808 total
$ time python so.py 4096 2600.txt.utf-8
python so.py 4096 2600.txt.utf-8  1.31s user 0.01s system 99% cpu 1.318 total

现在:不要将缓冲区大小4096视为普遍真理; 看看我得到的不同大小的结果(缓冲区大小(字节)与墙上时间(秒)):

   2 2.726 
   4 1.948 
   8 1.693 
  16 1.534 
  32 1.525 
  64 1.398 
 128 1.432 
 256 1.377 
 512 1.347 
1024 1.442 
2048 1.316 
4096 1.318 

正如你所看到的,你可以在早些时候开始看到收益(我的时间可能非常不准确); 缓冲区大小是性能和内存之间的权衡.默认值4096只是一个合理的选择,但与往常一样,先测量.

Answer 4

Python本身可以在交互模式下为您提供帮助:

>>> help(file.read)
Help on method_descriptor:

read(...)
    read([size]) -> read at most size bytes, returned as a string.

    If the size argument is negative or omitted, read until EOF is reached.
    Notice that when in non-blocking mode, less data than what was requested
    may be returned, even if no size parameter was given.

Answer 5

只是:

myfile = open(filename)
onecaracter = myfile.read(1)

Answer 6

我今天在观看 Raymond Hettinger 的Transforming Code into Beautiful, Idiomatic Python 时学到了一个新的习语：

import functools

with open(filename) as f:
    f_read_ch = functools.partial(f.read, 1)
    for ch in iter(f_read_ch, ''):
        print 'Read a character:', repr(ch)