Python 警告出现在试图警告用户的事情之后

Question

我在目标代码中使用警告来提醒用户发生了某些事情但不会停止代码。下面是基于我在实际代码中遇到的更复杂场景的简单模型：

from warnings import warn
class myClass(object):        
    def __init__(self, numberArg):

        if numberArg > 1000:
            self._tmpTxt = "That's a really big number for this code." + \
                           "Code may take a while to run..."
            warn("\n%s %s" %(numberArg, self._tmpTxt))
            print("If this were real code:")
            print("Actions code takes because this is a big number would happen here.")

        print("If this were real code, it would be doing more stuff here ...")

mc1 = myClass(1001)

在我的真实代码中，当我实例化执行__init__(self, numberArg)的class时，警告之后的所有处理完成后，最后输出警告。为什么会这样？

更重要的是，有没有办法确保首先输出警告，然后我的其余代码运行并提供其输出？

与此处提供的示例一样，预期的效果是在发生之前提醒用户将要发生的事情，而不是之后，并以警告格式提供类似于警告的输出。

注意：在 Windows 7 环境

上 iPython/Jupyter 使用 Python 2.7 时遇到此问题

Answer 1

@direprobs 在评论中为这个问题提供了最简单的答案。在调用 warn().

之后添加这行代码

sys.stderr.flush()

可以将此代码复制并粘贴到 Python 2.7（Jupyter Notebooks）中以快速运行并查看效果：

实验一（与后面的代码进行比较）：

# Note how warnings in this sample are held until after code is run and then output at the end ...

from warnings import warn
from warnings import resetwarnings

class myClass(object):        
    def __init__(self, numberArg):

        if numberArg > 1000:
            self._tmpTxt = "That's a really big number for this code." + \
                           "Code may take a while to run..."
            warn("\n%s %s" %(numberArg, self._tmpTxt), stacklevel=1, category=RuntimeWarning)
                                                       # possible categories (some of them):
                                                       # UserWarning, Warning, RunTimeWarning, ResourceWarning
                                                       # stacklevel was a experiment w/ no visible effect
                                                       # in this instance
            
            resetwarnings()                            # tried putting this before and after the warn()
            print("If this were real code:")
            print("Actions code takes because this is a big number would happen here.")
        
        print("If this were real code, it would be doing more stuff here ...")
        
mc1 = myClass(1001)

实验二：

# In this case, we want the warning to come before code execution.  This is easily fixed as shown below.
# note: removed some extraneous useless stuff, the line to look for is sys.stderr.flush()

from warnings import warn
from warnings import resetwarnings
import sys

class myClass(object):        
    def __init__(self, numberArg):

        if numberArg > 1000:
            self._tmpTxt = "That's a really big number for this code." + \
                           "Code may take a while to run..."
            warn("\n%s %s" %(numberArg, self._tmpTxt), category=Warning)            
            sys.stderr.flush()                         # put this after each warn() to make it output more immediately
            print("If this were real code:")
            print("Actions code takes because this is a big number would happen here.")
        
        print("If this were real code, it would be doing more stuff here ...")
        
mc1 = myClass(1001)

实验三：

# code provided as an experiment ... may be updated later with a more useful example ...
# in theory, filterwarnings should help shake out repeat warnings if used with right arguments
#   * note how our loop causes the content to print twice, and in theory, the 3 instances of warnings
#   * occur twice each for 6 possible output warnings
#   * each new occurance (3 of them) still outputs, but when the same ones come up again, they don't
#   * we get 3 instead of 6 warnings ... this should be the effect of filterwarning("once")
#     in this instance

# help on this: https://docs.python.org/3/library/warnings.html#warning-filter
#               in this example:
#                  "once" arg = print only the first occurrence of matching warnings, regardless of location

from warnings import warn
from warnings import resetwarnings
from warnings import filterwarnings

class myClass(object):        
    def __init__(self, numberArg):
        
        for i in [1,2]:

            if numberArg > 1000:
                print("loop count %d:" %(i))
                self._tmpTxt = "That's a really big number for this code." + \
                               "Code may take a while to run..."
                filterwarnings("once")
                warn("\n%s %s" %(numberArg, self._tmpTxt), stacklevel=1, category=RuntimeWarning)
                sys.stderr.flush() # this provides warning ahead of the output instead of after it
                # resetwarnings()  # no noticeable effect on the code
                print("If this were real code:")
                print("Actions code takes because this is a big number would happen here.")

            if numberArg > 20000:
                self._tmpTxt = "That's a really really really big number for this code." + \
                               "Code may take a while to run..."                
                filterwarnings("once", "\nFW: %s %s" %(numberArg, self._tmpTxt))
                warn("\n%s %s" %(numberArg, self._tmpTxt), stacklevel=0)
                # resetwarnings()  # no noticeable effect on the code
                sys.stderr.flush() # this provides warning ahead of the output instead of after it

            print("loop count %d:" %(i))    
            print("If this were real code, it would be doing more stuff here ...")

mc1 = myClass(1001)
print("====================")
mc2 = myClass(20001)

稍后在 github 上查找此代码。将它张贴在这里是为了帮助其他人调查如何使用 warnings.

Python 警告出现在试图警告用户的事情之后

Python warnings come after thing trying to warn user about

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class

ipython