我可以将python中的stdout重定向到某种字符串缓冲区吗？

我使用python ftplib编写了一个小型FTP客户端，但程序包中的某些函数不会返回字符串输出，而是输出到stdout。我想重定向stdout到一个我将能够从中读取输出的对象。

我知道stdout可以使用以下命令将其重定向到任何常规文件中：

stdout = open("file", "a")

但是我更喜欢不使用本地驱动器的方法。

我正在寻找类似BufferedReaderJava的东西，可用于将缓冲区包装到流中。

from cStringIO import StringIO # Python3 use: from io import StringIO
import sys

old_stdout = sys.stdout
sys.stdout = mystdout = StringIO()

# blah blah lots of code ...

sys.stdout = old_stdout

# examine mystdout.getvalue()

Python 3.4中有contextlib.redirect_stdout（）函数：

import io
from contextlib import redirect_stdout

with io.StringIO() as buf, redirect_stdout(buf):
    print('redirected')
    output = buf.getvalue()

以下代码示例显示了如何在旧版Python上实现它。

只是为了补充上述Ned的答案：您可以使用它将输出重定向到实现write（str）方法的任何对象。

这可以很好地用于在GUI应用程序中“捕获” stdout输出。

这是PyQt中一个愚蠢的例子：

import sys
from PyQt4 import QtGui

class OutputWindow(QtGui.QPlainTextEdit):
    def write(self, txt):
        self.appendPlainText(str(txt))

app = QtGui.QApplication(sys.argv)
out = OutputWindow()
sys.stdout=out
out.show()
print "hello world !"

从Python 2.6开始，您可以使用实现io模块中的TextIOBaseAPI的任何方法来代替。此解决方案还使您能够sys.stdout.buffer.write()在Python 3中使用（已）将编码的字节字符串写入stdout（请参阅Python 3中的stdout）。StringIO那时，使用将不起作用，因为sys.stdout.encoding也不sys.stdout.buffer可用。

使用TextIOWrapper的解决方案：

import sys
from io import TextIOWrapper, BytesIO

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

# do something that writes to stdout or stdout.buffer

# 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

此解决方案适用于Python 2> = 2.6和Python 3。

请注意，我们的新产品sys.stdout.write()仅接受unicode字符串，并且sys.stdout.buffer.write()仅接受字节字符串。对于旧代码而言，情况可能并非如此，但对于在Python 2和3上运行且无需更改的代码而言，情况往往如此sys.stdout.buffer。

您可以构建一个稍微的变化以接受unicode和byte字符串用于write()：

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，但这在使用此方法测试/比较脚本输出时会有所帮助。

即使存在异常，此方法也将还原sys.stdout。它还会在异常发生前获取任何输出。

import io
import sys

real_stdout = sys.stdout
fake_stdout = io.BytesIO()   # or perhaps io.StringIO()
try:
    sys.stdout = fake_stdout
    # do what you have to do to create some output
finally:
    sys.stdout = real_stdout
    output_string = fake_stdout.getvalue()
    fake_stdout.close()
    # do what you want with the output_string

使用Python 2.7.10测试 io.BytesIO()

使用Python 3.6.4进行了测试 io.StringIO()

鲍勃（Bob），添加了一个案例，如果您感觉到修改/扩展代码实验中的任何内容，可能会在某种意义上变得有趣，否则可以将其删除

^{广告信息...在寻找一些可行的机制来“抓取”输出的过程中，通过扩展实验的一些评论，numexpr.print_versions()直接针对<stdout>（需要清理GUI并将详细信息收集到调试报告中）}

# THIS WORKS AS HELL: as Bob Stein proposed years ago:
#  py2 SURPRISEDaBIT:
#
import io
import sys
#
real_stdout = sys.stdout                        #           PUSH <stdout> ( store to REAL_ )
fake_stdout = io.BytesIO()                      #           .DEF FAKE_
try:                                            # FUSED .TRY:
    sys.stdout.flush()                          #           .flush() before
    sys.stdout = fake_stdout                    #           .SET <stdout> to use FAKE_
    # ----------------------------------------- #           +    do what you gotta do to create some output
    print 123456789                             #           + 
    import  numexpr                             #           + 
    QuantFX.numexpr.__version__                 #           + [3] via fake_stdout re-assignment, as was bufferred + "late" deferred .get_value()-read into print, to finally reach -> real_stdout
    QuantFX.numexpr.print_versions()            #           + [4] via fake_stdout re-assignment, as was bufferred + "late" deferred .get_value()-read into print, to finally reach -> real_stdout
    _ = os.system( 'echo os.system() redir-ed' )#           + [1] via real_stdout                                 + "late" deferred .get_value()-read into print, to finally reach -> real_stdout, if not ( _ = )-caught from RET-d "byteswritten" / avoided from being injected int fake_stdout
    _ = os.write(  sys.stderr.fileno(),         #           + [2] via      stderr                                 + "late" deferred .get_value()-read into print, to finally reach -> real_stdout, if not ( _ = )-caught from RET-d "byteswritten" / avoided from being injected int fake_stdout
                       b'os.write()  redir-ed' )#  *OTHERWISE, if via fake_stdout, EXC <_io.BytesIO object at 0x02C0BB10> Traceback (most recent call last):
    # ----------------------------------------- #           ?                              io.UnsupportedOperation: fileno
    #'''                                                    ? YET:        <_io.BytesIO object at 0x02C0BB10> has a .fileno() method listed
    #>>> 'fileno' in dir( sys.stdout )       -> True        ? HAS IT ADVERTISED,
    #>>> pass;            sys.stdout.fileno  -> <built-in method fileno of _io.BytesIO object at 0x02C0BB10>
    #>>> pass;            sys.stdout.fileno()-> Traceback (most recent call last):
    #                                             File "<stdin>", line 1, in <module>
    #                                           io.UnsupportedOperation: fileno
    #                                                       ? BUT REFUSES TO USE IT
    #'''
finally:                                        # == FINALLY:
    sys.stdout.flush()                          #           .flush() before ret'd back REAL_
    sys.stdout = real_stdout                    #           .SET <stdout> to use POP'd REAL_
    sys.stdout.flush()                          #           .flush() after  ret'd back REAL_
    out_string = fake_stdout.getvalue()         #           .GET string           from FAKE_
    fake_stdout.close()                         #                <FD>.close()
    # +++++++++++++++++++++++++++++++++++++     # do what you want with the out_string
    #
    print "\n{0:}\n{1:}{0:}".format( 60 * "/\\",# "LATE" deferred print the out_string at the very end reached -> real_stdout
                                     out_string #                   
                                     )
'''
PASS'd:::::
...
os.system() redir-ed
os.write()  redir-ed
/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\
123456789
'2.5'
-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=
Numexpr version:   2.5
NumPy version:     1.10.4
Python version:    2.7.13 |Anaconda 4.0.0 (32-bit)| (default, May 11 2017, 14:07:41) [MSC v.1500 32 bit (Intel)]
AMD/Intel CPU?     True
VML available?     True
VML/MKL version:   Intel(R) Math Kernel Library Version 11.3.1 Product Build 20151021 for 32-bit applications
Number of threads used by default: 4 (out of 4 detected cores)
-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=
/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\
>>>

EXC'd :::::
...
os.system() redir-ed
/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\
123456789
'2.5'
-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=
Numexpr version:   2.5
NumPy version:     1.10.4
Python version:    2.7.13 |Anaconda 4.0.0 (32-bit)| (default, May 11 2017, 14:07:41) [MSC v.1500 32 bit (Intel)]
AMD/Intel CPU?     True
VML available?     True
VML/MKL version:   Intel(R) Math Kernel Library Version 11.3.1 Product Build 20151021 for 32-bit applications
Number of threads used by default: 4 (out of 4 detected cores)
-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=
/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\

Traceback (most recent call last):
  File "<stdin>", line 9, in <module>
io.UnsupportedOperation: fileno
'''

python3的上下文管理器：

import sys
from io import StringIO


class RedirectedStdout:
    def __init__(self):
        self._stdout = None
        self._string_io = None

    def __enter__(self):
        self._stdout = sys.stdout
        sys.stdout = self._string_io = StringIO()
        return self

    def __exit__(self, type, value, traceback):
        sys.stdout = self._stdout

    def __str__(self):
        return self._string_io.getvalue()

像这样使用：

>>> with RedirectedStdout() as out:
>>>     print('asdf')
>>>     s = str(out)
>>>     print('bsdf')
>>> print(s, out)
'asdf\n' 'asdf\nbsdf\n'

在Python3.6中，StringIOand cStringIO模块不见了，您应该改用，所以您应该io.StringIO像第一个答案那样进行操作：

import sys
from io import StringIO

old_stdout = sys.stdout
old_stderr = sys.stderr
my_stdout = sys.stdout = StringIO()
my_stderr = sys.stderr = StringIO()

# blah blah lots of code ...

sys.stdout = self.old_stdout
sys.stderr = self.old_stderr

// if you want to see the value of redirect output, be sure the std output is turn back
print(my_stdout.getvalue())
print(my_stderr.getvalue())

my_stdout.close()
my_stderr.close()

使用pipe()并写入适当的文件描述符。

https://docs.python.org/library/os.html#file-descriptor-operations

这是另一种看法。 contextlib.redirect_stdout与io.StringIO()作为记录的是伟大的，但它仍然是一个有点冗长，日常使用。这是通过子类化使其成为单线的方法contextlib.redirect_stdout：

import sys
import io
from contextlib import redirect_stdout

class capture(redirect_stdout):

    def __init__(self):
        self.f = io.StringIO()
        self._new_target = self.f
        self._old_targets = []  # verbatim from parent class

    def __enter__(self):
        self._old_targets.append(getattr(sys, self._stream))  # verbatim from parent class
        setattr(sys, self._stream, self._new_target)  # verbatim from parent class
        return self  # instead of self._new_target in the parent class

    def __repr__(self):
        return self.f.getvalue()  

由于__enter__返回self，因此在with块退出之后，可以使用上下文管理器对象。而且，由于使用__repr__方法，上下文管理器对象的字符串表示实际上是stdout。所以现在你有了

with capture() as message:
    print('Hello World!')
print(str(message)=='Hello World!\n')  # returns True