110

假设有一个脚本在做这样的事情:

# module writer.py
import sys

def write():
    sys.stdout.write("foobar")

现在假设我想捕获write函数的输出并将其存储在一个变量中以供进一步处理。天真的解决方案是:

# module mymodule.py
from writer import write

out = write()
print out.upper()

但这不起作用。我想出了另一种解决方案并且它有效,但是请让我知道是否有更好的方法来解决问题。谢谢

import sys
from cStringIO import StringIO

# setup the environment
backup = sys.stdout

# ####
sys.stdout = StringIO()     # capture output
write()
out = sys.stdout.getvalue() # release output
# ####

sys.stdout.close()  # close the stream 
sys.stdout = backup # restore original stdout

print out.upper()   # post processing
4

11 回答 11

65

对于未来的访问者:Python 3.4 contextlib 通过上下文管理器直接提供了这一点(请参阅Python contextlib 帮助redirect_stdout

from contextlib import redirect_stdout
import io

f = io.StringIO()
with redirect_stdout(f):
    help(pow)
s = f.getvalue()
于 2016-11-04T07:25:38.240 回答
55

设置stdout是一种合理的方法。另一种是将其作为另一个进程运行:

import subprocess

proc = subprocess.Popen(["python", "-c", "import writer; writer.write()"], stdout=subprocess.PIPE)
out = proc.communicate()[0]
print out.upper()
于 2011-02-27T23:02:39.753 回答
52

这是您的代码的上下文管理器版本。它产生一个包含两个值的列表;第一个是标准输出,第二个是标准错误。

import contextlib
@contextlib.contextmanager
def capture():
    import sys
    from cStringIO import StringIO
    oldout,olderr = sys.stdout, sys.stderr
    try:
        out=[StringIO(), StringIO()]
        sys.stdout,sys.stderr = out
        yield out
    finally:
        sys.stdout,sys.stderr = oldout, olderr
        out[0] = out[0].getvalue()
        out[1] = out[1].getvalue()

with capture() as out:
    print 'hi'
于 2012-05-24T18:49:31.623 回答
14

从 Python 3 开始,您还可以使用sys.stdout.buffer.write()将(已经)编码的字节字符串写入标准输出(参见Python 3 中的标准输出)。当你这样做时,简单的StringIO方法不起作用,因为既不可用sys.stdout.encoding也不sys.stdout.buffer可用。

从 Python 2.6 开始,您可以使用TextIOBaseAPI,其中包括缺少的属性:

import sys
from io import TextIOWrapper, BytesIO

# setup the environment
old_stdout = sys.stdout
sys.stdout = TextIOWrapper(BytesIO(), sys.stdout.encoding)

# do some writing (indirectly)
write("blub")

# get output
sys.stdout.seek(0)      # jump to the start
out = sys.stdout.read() # read output

# restore stdout
sys.stdout.close()
sys.stdout = old_stdout

# do stuff with the output
print(out.upper())

此解决方案适用于 Python 2 >= 2.6 和 Python 3。请注意,我们sys.stdout.write()只接受 unicode 字符串并且sys.stdout.buffer.write()只接受字节字符串。旧代码可能不是这种情况,但通常是为在 Python 2 和 3 上运行而无需更改而构建的代码的情况。

如果您需要支持不使用 stdout.buffer 直接将字节字符串发送到 stdout 的代码,则可以使用以下变体:

class StdoutBuffer(TextIOWrapper):
    def write(self, string):
        try:
            return super(StdoutBuffer, self).write(string)
        except TypeError:
            # redirect encoded byte strings directly to buffer
            return super(StdoutBuffer, self).buffer.write(string)

您不必将缓冲区的编码设置为 sys.stdout.encoding,但这有助于使用此方法测试/比较脚本输出。

于 2013-10-13T11:52:08.447 回答
10

这是我原始代码的装饰器对应物。

writer.py保持不变:

import sys

def write():
    sys.stdout.write("foobar")

mymodule.py稍微修改一下:

from writer import write as _write
from decorators import capture

@capture
def write():
    return _write()

out = write()
# out post processing...

这是装饰器:

def capture(f):
    """
    Decorator to capture standard output
    """
    def captured(*args, **kwargs):
        import sys
        from cStringIO import StringIO

        # setup the environment
        backup = sys.stdout

        try:
            sys.stdout = StringIO()     # capture output
            f(*args, **kwargs)
            out = sys.stdout.getvalue() # release output
        finally:
            sys.stdout.close()  # close the stream 
            sys.stdout = backup # restore original stdout

        return out # captured output wrapped in a string

    return captured
于 2011-03-16T00:17:30.973 回答
9

或者也许使用已经存在的功能......

from IPython.utils.capture import capture_output

with capture_output() as c:
    print('some output')

c()

print c.stdout
于 2016-02-25T10:18:12.680 回答
6

这是一个上下文管理器,其灵感来自@JonnyJD 的回答,支持将字节写入buffer属性,但也利用sys 的 dunder-io 引用来进一步简化。

import io
import sys
import contextlib


@contextlib.contextmanager
def capture_output():
    output = {}
    try:
        # Redirect
        sys.stdout = io.TextIOWrapper(io.BytesIO(), sys.stdout.encoding)
        sys.stderr = io.TextIOWrapper(io.BytesIO(), sys.stderr.encoding)
        yield output
    finally:
        # Read
        sys.stdout.seek(0)
        sys.stderr.seek(0)
        output['stdout'] = sys.stdout.read()
        output['stderr'] = sys.stderr.read()
        sys.stdout.close()
        sys.stderr.close()

        # Restore
        sys.stdout = sys.__stdout__
        sys.stderr = sys.__stderr__


with capture_output() as output:
    print('foo')
    sys.stderr.buffer.write(b'bar')

print('stdout: {stdout}'.format(stdout=output['stdout']))
print('stderr: {stderr}'.format(stderr=output['stderr']))

输出是:

stdout: foo

stderr: bar
于 2020-10-10T03:31:01.957 回答
4

我认为你应该看看这四个对象:

from test.test_support import captured_stdout, captured_output, \
    captured_stderr, captured_stdin

例子:

from writer import write

with captured_stdout() as stdout:
    write()
print stdout.getvalue().upper()

UPD:正如 Eric 在评论中所说,不应该直接使用它们,所以我复制并粘贴了它。

# Code from test.test_support:
import contextlib
import sys

@contextlib.contextmanager
def captured_output(stream_name):
    """Return a context manager used by captured_stdout and captured_stdin
    that temporarily replaces the sys stream *stream_name* with a StringIO."""
    import StringIO
    orig_stdout = getattr(sys, stream_name)
    setattr(sys, stream_name, StringIO.StringIO())
    try:
        yield getattr(sys, stream_name)
    finally:
        setattr(sys, stream_name, orig_stdout)

def captured_stdout():
    """Capture the output of sys.stdout:

       with captured_stdout() as s:
           print "hello"
       self.assertEqual(s.getvalue(), "hello")
    """
    return captured_output("stdout")

def captured_stderr():
    return captured_output("stderr")

def captured_stdin():
    return captured_output("stdin")
于 2013-09-17T08:14:14.390 回答
4

这里的问题(如何重定向输出的示例,而不是tee部分)用于os.dup2在操作系统级别重定向流。这很好,因为它也适用于您从程序中生成的命令。

于 2011-02-27T22:54:20.560 回答
4

我喜欢 contextmanager 解决方案,但是如果您需要使用打开的文件和 fileno 支持存储的缓冲区,您可以执行类似的操作。

import six
from six.moves import StringIO


class FileWriteStore(object):
    def __init__(self, file_):
        self.__file__ = file_
        self.__buff__ = StringIO()

    def __getattribute__(self, name):
        if name in {
            "write", "writelines", "get_file_value", "__file__",
                "__buff__"}:
            return super(FileWriteStore, self).__getattribute__(name)
        return self.__file__.__getattribute__(name)

    def write(self, text):
        if isinstance(text, six.string_types):
            try:
                self.__buff__.write(text)
            except:
                pass
        self.__file__.write(text)

    def writelines(self, lines):
        try:
            self.__buff__.writelines(lines)
        except:
            pass
        self.__file__.writelines(lines)

    def get_file_value(self):
        return self.__buff__.getvalue()

利用

import sys
sys.stdout = FileWriteStore(sys.stdout)
print "test"
buffer = sys.stdout.get_file_value()
# you don't want to print the buffer while still storing
# else it will double in size every print
sys.stdout = sys.stdout.__file__
print buffer
于 2016-10-11T15:24:40.770 回答
0

第三方代码已经复制了引用的另一种方法sys.stdout是临时替换write()方法本身:

from types import MethodType
...
f = io.StringIO()
def new_write(self, data):
    f.write(data)

old_write = sys.stdout.write
sys.stdout.write = MethodType(new_write, sys.stdout)
error = command.run(args)
sys.stdout.write = old_write
output = f.getvalue()
于 2022-02-04T21:09:29.980 回答