我可以将标准输出重定向到某种字符串缓冲区吗？

Question

我正在使用 python 的 ftplib 编写一个小型 FTP 客户端，但包中的某些函数不返回字符串输出，而是打印到 stdout。 我想将 stdout 重定向到一个我能够读取输出的对象。

我知道 stdout 可以通过以下方式重定向到任何常规文件：

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

但我更喜欢不使用本地驱动器的方法。

我正在寻找类似 Java 中的 BufferedReader 之类的东西，可用于将缓冲区包装到流中。

Answer 1

这是对此的另一种看法。 contextlib.redirect_stdout 与 io.StringIO() 作为 记录很棒，但对于日常使用来说仍然有点冗长。 以下是如何通过子类化 contextlib.redirect_stdout 使其成为单行代码：

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 块退出后可以使用上下文管理器对象。 此外，由于 __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，

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

Answer 2

使用pipe()并写入适当的文件描述符。

https://docs.python.org/library/os.html#文件描述符操作

Answer 3

在Python3.6中，StringIO和cStringIO模块已经消失，你应该使用io.StringIO代替。所以你应该像第一个一样这样做回答：

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()

Answer 4

即使出现异常，此方法也会恢复 sys.stdout。 它还会在异常发生之前获取任何输出。

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 io.BytesIO()< 在 Python 2.7.10 中进行测试/code>

使用 io 在 Python 3.6.4 中进行测试.StringIO()

---

Bob，如果您觉得修改/扩展的代码实验中的任何内容可能会在任何意义上变得有趣，则添加一个案例，否则随意删除它

^{Ad informandum ...在寻找一些可行的机制来“抓取”输出期间进行扩展实验的一些评论，由 numexpr.print_versions() 直接定向到 <; 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
'''

Answer 5

从 Python 2.6 开始，您可以使用任何实现 io.TextIOBase API 作为替代品。
此解决方案还允许您在 Python 3 中使用 sys.stdout.buffer.write() 来将（已）编码的字节字符串写入标准输出（请参阅 Python 3 中的 stdout）。
那么使用 io.StringIO 就不起作用了，因为 sys.stdout.buffer 和 sys.stdout.encoding 都不可用。

使用 io.TextIOWrapper 的解决方案：

import io
import sys

# Setup stdout.
old_stdout = sys.stdout
sys.stdout = io.TextIOWrapper(io.BytesIO(), sys.stdout.encoding)

# Write to stdout or stdout.buffer.
...

# Read from stdout.
sys.stdout.seek(0)
out = sys.stdout.read()

# 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，新的 sys.stdout.write() 只接受 unicode字符串和 sys.stdout.buffer.write() 只接受字节字符串。
无需更改即可在 Python 2 和 3 上运行的代码通常会使用 sys.stdout.buffer。

因此，要让 sys.stdout.write() 接受 unicode 和字节字符串，您可以使用这个 io.TextIOWrapper 子类：

class StdoutBuffer(io.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)

您不必设置编码缓冲区的 sys.stdout.encoding，但这在使用此方法测试/比较脚本输出时很有帮助。

Answer 6

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()

使用如下：

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

Answer 7

只是添加到上面 Ned 的答案中：您可以使用它将输出重定向到任何实现 write(str) 方法的对象。

这可以很好地用于在 GUI 应用程序中“捕获”stdout 输出。

这是 PyQt 中的一个愚蠢的例子：

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 !"

Answer 8

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()

Answer 9

有一个 contextlib.redirect_stdout() 函数< Python 3.4+ 中的 /a>：

import io
from contextlib import redirect_stdout

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

这是一个代码示例，展示了如何在较旧的 Python 版本上实现它。