我正在使用python的ftplib编写一个小型FTP客户端，但是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重定向到一个对象，我将能够从中读取输出。
我知道stdout可以通过以下方式重定向到任何常规文件中：
stdout = open("file", "a")
但我更喜欢不使用本地驱动器的方法。
我正在寻找像Java中的BufferedReader这样的东西，可用于将缓冲区包装到流中。
答案 0 :(得分：171)
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()
答案 1 :(得分：54)
Python 3.4中有contextlib.redirect_stdout() function：
import io
from contextlib import redirect_stdout
with io.StringIO() as buf, redirect_stdout(buf):
print('redirected')
output = buf.getvalue()
此处code example that shows how to implement it on older Python versions。
答案 2 :(得分：34)
只需添加上面的Ned答案：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 !"
答案 3 :(得分：6)
从Python 2.6开始，您可以使用io模块中实现TextIOBase API的任何内容作为替代。
此解决方案还允许您使用Python 3中的sys.stdout.buffer.write()将已编码的字节字符串写入stdout（请参阅stdout in Python 3）。
然后使用StringIO将无效，因为sys.stdout.encoding和sys.stdout.buffer都不可用。
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＆gt; = 2.6和Python 3。
请注意，我们的新sys.stdout.write()只接受unicode字符串，sys.stdout.buffer.write()只接受字节字符串。
对于旧代码可能不是这种情况，但是对于构建为在没有更改的情况下在Python 2和3上运行的代码通常就是这种情况，这通常也会使用sys.stdout.buffer。
您可以构建一个轻微的变体，接受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。
答案 4 :(得分：5)
即使存在异常，此方法也会恢复sys.stdout。它还会在异常之前获得任何输出。
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
鲍勃，如果您觉得修改/扩展代码实验中的任何内容可能会在任何意义上变得有趣，请添加一个案例，否则随意删除它
广告信息...在寻找一些可行的机制以及“抓住”时，通过扩展实验的一些评论。输出，由
numexpr.print_versions()直接指向<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
'''
答案 5 :(得分：4)
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答案那样做：
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()
答案 6 :(得分：3)
使用pipe()并写入相应的文件描述符。
https://docs.python.org/library/os.html#file-descriptor-operations
答案 7 :(得分：2)
python3的上下文管理器：
myget1 : (WebData HttpBin -> msg) -> Cmd msg
像这样使用：
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()
答案 8 :(得分：1)
这是另一种看法。以contextlib.redirect_stdout为documented的io.StringIO()很好，但是每天使用仍然有些冗长。通过子类化contextlib.redirect_stdout使其成为单线的方法如下：
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块退出之后您可以使用上下文管理器对象。此外，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。所以现在有了，
with capture() as message:
print('Hello World!')
print(str(message)=='Hello World!\n') # returns True