在Python中,给定一个文件的名称,如何编写一个循环,每次通过循环读取一个字符?
with open(filename) as f:
while True:
c = f.read(1)
if not c:
print("End of file")
break
print("Read a character:", c)
首先,打开一个文件:
with open("filename") as fileobj:
for line in fileobj:
for ch in line:
print(ch)
此代码逐行读取文件中的每一行,然后遍历该行中的每个字符。
我喜欢被接受的答案:它简单明了且能够完成任务。我也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))
我建议的代码本质上与您接受的答案相同:从文件中读取指定数量的字节。不同之处在于它首先读取一块好的数据(4006对于X86来说是一个很好的默认值,但您可能想尝试1024或8192;任何页面大小的倍数),然后一次一个地生成该块中的字符。
我提出的代码可能对于较大的文件更快。例如,以托尔斯泰的《战争与和平》整篇文本为例。这是我的计时结果(Mac Book Pro使用OS X 10.7.4; so.py是我给粘贴的代码起的名字):
$ 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,请不要把缓冲区大小 4096
视为普遍真理;看一下我对于不同大小所得到的结果(缓冲区大小(字节)与墙壁时间(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,你可以更早地开始看到收益(而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。4096的默认值只是一个合理的选择,但始终要先进行测量。
只需:
myfile = open(filename)
onecharacter = myfile.read(1)
Python本身可以在交互模式下帮助您完成此操作:
>>> 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.
今天我在观看 Raymond Hettinger 的 将代码转换成美丽、惯用的 Python 视频时学到了一个新的成语:
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)
Python 3.8+的最佳答案:
with open(path, encoding="utf-8") as f:
while c := f.read(1):
do_my_thing(c)
您可能希望指定utf-8编码并避免使用平台编码。我选择在此处执行此操作。
功能 - Python 3.8+:
def stream_file_chars(path: str):
with open(path) as f:
while c := f.read(1):
yield c
函数 - Python<=3.7:
def stream_file_chars(path: str):
with open(path, encoding="utf-8") as f:
while True:
c = f.read(1)
if c == "":
break
yield c
Function – pathlib + documentation:
from pathlib import Path
from typing import Union, Generator
def stream_file_chars(path: Union[str, Path]) -> Generator[str, None, None]:
"""Streams characters from a file."""
with Path(path).open(encoding="utf-8") as f:
while (c := f.read(1)) != "":
yield c
只读取一个字符
f.read(1)
with open("filename") as fileObj:
for line in fileObj:
for ch in line:
print(ch)
它会遍历文件中的每一行,以及每一行中的每个字符。
(请注意,尽管现在这篇文章看起来与一个高赞答案非常相似,但在撰写时并非如此。)
f.read(1)
,这绝对是正确的做法。
result = open(filename).read()
然后逐个字符读取result
。 - Shravanwith open(filename, encoding='Windows-1250') as f:
- SergOopen(filename, "r")
和open(filename, "rb")
之间的区别可能会导致不同数量的迭代(至少在Python 3中是这样)。如果遇到适当的特殊字符,“r”模式可以读取多个字节以获取c
。 - dcc310