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。下面是一个基于我在真实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) 的类时，在警告之后的所有处理完成后，最后会输出警告。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，所需的效果是在发生之前而不是之后提醒用户将要发生的事情，并像带有警告格式的警告一样提供输出。

注意：此问题是在 Windows 7 环境中的 iPython/Jupyter 上使用 Python 2.7 时遇到的

Answer 1

@direprobs 在 cmets 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（用于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。在此处发布以帮助其他人研究如何使用warnings。