我也有类似的情况。不清楚你是否知道以字节为单位的块大小;我通常不这样做，但所需要的记录(行)的数量是已知的:

def get_line():
     with open('4gb_file') as file:
         for i in file:
             yield i

lines_required = 100
gen = get_line()
chunk = [i for i, j in zip(gen, range(lines_required))]

更新:谢谢nosklo。这就是我的意思。它几乎工作，除了它丢失了一行“之间”块。

chunk = [next(gen) for i in range(lines_required)]

做的把戏w/o失去任何线条，但它看起来不太好。

已经有很多好的答案，但是如果您的整个文件都在一行上，并且您仍然想处理“行”(而不是固定大小的块)，那么这些答案对您没有帮助。

99%的情况下，可以逐行处理文件。然后，正如回答中所建议的，你可以使用文件对象本身作为惰性生成器:

with open('big.csv') as f:
    for line in f:
        process(line)

但是，可能会遇到行分隔符不是'\n'(常见情况是'|')的非常大的文件。

在处理之前将'|'转换为'\n'可能不是一个选项，因为它可能会混淆可能合法包含'\n'的字段(例如自由文本用户输入)。 使用csv库也被排除在外，因为至少在lib的早期版本中，它是硬编码来逐行读取输入的。

对于这种情况，我创建了以下代码段[在2021年5月针对Python 3.8+更新]:

def rows(f, chunksize=1024, sep='|'):
    """
    Read a file where the row separator is '|' lazily.

    Usage:

    >>> with open('big.csv') as f:
    >>>     for r in rows(f):
    >>>         process(r)
    """
    row = ''
    while (chunk := f.read(chunksize)) != '':   # End of file
        while (i := chunk.find(sep)) != -1:     # No separator found
            yield row + chunk[:i]
            chunk = chunk[i+1:]
            row = ''
        row += chunk
    yield row

[对于较旧版本的python]:

def rows(f, chunksize=1024, sep='|'):
    """
    Read a file where the row separator is '|' lazily.

    Usage:

    >>> with open('big.csv') as f:
    >>>     for r in rows(f):
    >>>         process(r)
    """
    curr_row = ''
    while True:
        chunk = f.read(chunksize)
        if chunk == '': # End of file
            yield curr_row
            break
        while True:
            i = chunk.find(sep)
            if i == -1:
                break
            yield curr_row + chunk[:i]
            curr_row = ''
            chunk = chunk[i+1:]
        curr_row += chunk

我能够成功地使用它来解决各种问题。它已经通过了各种块大小的广泛测试。以下是我正在使用的测试套件，供那些需要说服自己的人使用:

test_file = 'test_file'

def cleanup(func):
    def wrapper(*args, **kwargs):
        func(*args, **kwargs)
        os.unlink(test_file)
    return wrapper

@cleanup
def test_empty(chunksize=1024):
    with open(test_file, 'w') as f:
        f.write('')
    with open(test_file) as f:
        assert len(list(rows(f, chunksize=chunksize))) == 1

@cleanup
def test_1_char_2_rows(chunksize=1024):
    with open(test_file, 'w') as f:
        f.write('|')
    with open(test_file) as f:
        assert len(list(rows(f, chunksize=chunksize))) == 2

@cleanup
def test_1_char(chunksize=1024):
    with open(test_file, 'w') as f:
        f.write('a')
    with open(test_file) as f:
        assert len(list(rows(f, chunksize=chunksize))) == 1

@cleanup
def test_1025_chars_1_row(chunksize=1024):
    with open(test_file, 'w') as f:
        for i in range(1025):
            f.write('a')
    with open(test_file) as f:
        assert len(list(rows(f, chunksize=chunksize))) == 1

@cleanup
def test_1024_chars_2_rows(chunksize=1024):
    with open(test_file, 'w') as f:
        for i in range(1023):
            f.write('a')
        f.write('|')
    with open(test_file) as f:
        assert len(list(rows(f, chunksize=chunksize))) == 2

@cleanup
def test_1025_chars_1026_rows(chunksize=1024):
    with open(test_file, 'w') as f:
        for i in range(1025):
            f.write('|')
    with open(test_file) as f:
        assert len(list(rows(f, chunksize=chunksize))) == 1026

@cleanup
def test_2048_chars_2_rows(chunksize=1024):
    with open(test_file, 'w') as f:
        for i in range(1022):
            f.write('a')
        f.write('|')
        f.write('a')
        # -- end of 1st chunk --
        for i in range(1024):
            f.write('a')
        # -- end of 2nd chunk
    with open(test_file) as f:
        assert len(list(rows(f, chunksize=chunksize))) == 2

@cleanup
def test_2049_chars_2_rows(chunksize=1024):
    with open(test_file, 'w') as f:
        for i in range(1022):
            f.write('a')
        f.write('|')
        f.write('a')
        # -- end of 1st chunk --
        for i in range(1024):
            f.write('a')
        # -- end of 2nd chunk
        f.write('a')
    with open(test_file) as f:
        assert len(list(rows(f, chunksize=chunksize))) == 2

if __name__ == '__main__':
    for chunksize in [1, 2, 4, 8, 16, 32, 64, 128, 256, 512, 1024]:
        test_empty(chunksize)
        test_1_char_2_rows(chunksize)
        test_1_char(chunksize)
        test_1025_chars_1_row(chunksize)
        test_1024_chars_2_rows(chunksize)
        test_1025_chars_1026_rows(chunksize)
        test_2048_chars_2_rows(chunksize)
        test_2049_chars_2_rows(chunksize)

在Python 3.8+中，你可以在while循环中使用.read():

with open("somefile.txt") as f:
    while chunk := f.read(8192):
        do_something(chunk)

当然，你可以使用任何你想要的块大小，你不需要使用8192(2**13)字节。除非你的文件大小恰好是你的块大小的倍数，否则最后一个块将小于你的块大小。

由于我的低声誉，我不允许评论，但SilentGhosts解决方案应该更容易与file.readlines([sizehint])

Python文件方法

编辑:SilentGhost是对的，但这应该比:

s = "" 
for i in xrange(100): 
   s += file.next()

如果您的计算机、操作系统和python是64位的，那么您可以使用mmap模块将文件的内容映射到内存中，并使用索引和切片访问它。下面是文档中的一个例子:

import mmap
with open("hello.txt", "r+") as f:
    # memory-map the file, size 0 means whole file
    map = mmap.mmap(f.fileno(), 0)
    # read content via standard file methods
    print map.readline()  # prints "Hello Python!"
    # read content via slice notation
    print map[:5]  # prints "Hello"
    # update content using slice notation;
    # note that new content must have same size
    map[6:] = " world!\n"
    # ... and read again using standard file methods
    map.seek(0)
    print map.readline()  # prints "Hello  world!"
    # close the map
    map.close()

如果你的计算机、操作系统或python是32位的，那么映射大文件会占用你的大部分地址空间，并耗尽程序的内存。

f = ... # file-like object, i.e. supporting read(size) function and 
        # returning empty string '' when there is nothing to read

def chunked(file, chunk_size):
    return iter(lambda: file.read(chunk_size), '')

for data in chunked(f, 65536):
    # process the data

更新:该方法最好在https://stackoverflow.com/a/4566523/38592中解释