为什么功能正常工作而不指定参数？ [英] Why function works properly without specifying parameters?

问题描述

在下面的代码中，我不明白为什么 download_progress_hook 在从 maybe_download $ b

download_progress_hook 的定义表明有三个参数需要传递： code> count，blockSize，totalSize 。
但是，当从 maybe_download 调用 download_progress_hook 时，没有参数传递。为什么它不会失败？

以下是完整的代码：

  url ='http://commondatastorage.googleapis.com/books1000/'
 last_percent_reported = None 
 data_root ='。'＃改变我在别处存储数据
 
 def download_progress_hook（count，blockSize，totalSize）：
一个钩子报告下载进度，主要用于
网速较慢的用户，下载进度每下降5％。 

全局last_percent_reported 
百分比= int（count * blockSize * 100 / totalSize）
 
如果last_percent_reported！=百分比：
如果百分比％ 5 == 0：
 sys.stdout.write（％s %%％percent）
 sys.stdout.flush（）
 else：
 sys.stdout.write （。）
 sys.stdout.flush（）
 
 last_percent_reported = percent 
 $ b $ def maybe_download（filename，expected_bytes，force = False）：
如果文件不存在，请下载文件，并制作su 
 dest_filename = os.path.join（data_root，filename）
如果强制或不强制os.path.exists（dest_filename）：
 print（'Attempting下载：'，文件名）
文件名，_ = urlretrieve（url +文件名，dest_filename，reporthook = download_progress_hook）
 print（'\\\
Download Complete！'）
 statinfo = os.stat （dest_filename）
 if statinfo.st_size == expected_bytes：
 print（'Found and verified'，dest_filename）
 else：
 raise Exception（
'Failed to verify '+ dest_filename +'。 '）
 return dest_filename 
 
 train_filename = maybe_download（'notMNIST_large.tar.gz'，247336696）
 test_filename = maybe_download（'notMNIST_small。 tar.gz'，8458043）
  

解决方案

我得到了一切，但函数 download_progress_hook 从函数 maybe_download 中调用

那是你出错的地方。该功能未被调用。它只被引用。这里没有（...）调用表达式。

Python函数是一流的对象，你可以通过它们或将它们分配给其他名称：

 >>> def foo（bar）：
 ... return bar + 1 
 ... 
>>> foo 
<函数0x00e20410处的foo> 
>>> spam = foo 
>>>垃圾邮件
<函数foo在0x100e20410> 
>>>垃圾邮件（5）
 6 
  

这里 spam 是对函数对象 foo 的另一个引用。

所以下面的表达式：

 
  urlretrieve（
 url + filename，dest_filename，
 reporthook = download_progress_hook）
  

不会调用 download_progress_hook 。它只是将该函数对象赋给 urlretrieve（）函数，它是代码，它将调用 download_progress_hook somewhere（传入所需参数）。

从 URLOpener.retrieve 文档（最终处理该钩子）：
$ b

如果给出 reporthook ，它必须是一个接受三个数字参数的函数：块数，最大值大小块被读入并且下载的总大小（如果未知，则为-1）。它将在开始时被调用一次，并且在从网络读取每个数据块之后。

In the below code, I don't understand why download_progress_hook works without passing parameters when it is called from within the maybe_download method.

The definition of download_progress_hook states that there are three parameters that have to be passed: count, blockSize, totalSize. But, when the download_progress_hook is called from maybe_download there are no parameters passed. Why doesn't it fail?

Here is the full code:

url = 'http://commondatastorage.googleapis.com/books1000/'
last_percent_reported = None
data_root = '.' # Change me to store data elsewhere

def download_progress_hook(count, blockSize, totalSize):
  """A hook to report the progress of a download. This is mostly intended for users with
  slow internet connections. Reports every 5% change in download progress.
  """
  global last_percent_reported
  percent = int(count * blockSize * 100 / totalSize)

  if last_percent_reported != percent:
    if percent % 5 == 0:
      sys.stdout.write("%s%%" % percent)
      sys.stdout.flush()
    else:
      sys.stdout.write(".")
      sys.stdout.flush()

    last_percent_reported = percent

def maybe_download(filename, expected_bytes, force=False):
  """Download a file if not present, and make sure it's the right size."""
  dest_filename = os.path.join(data_root, filename)
  if force or not os.path.exists(dest_filename):
    print('Attempting to download:', filename) 
    filename, _ = urlretrieve(url + filename, dest_filename, reporthook=download_progress_hook)
    print('\nDownload Complete!')
  statinfo = os.stat(dest_filename)
  if statinfo.st_size == expected_bytes:
    print('Found and verified', dest_filename)
  else:
    raise Exception(
      'Failed to verify ' + dest_filename + '. Can you get to it with a browser?')
  return dest_filename

train_filename = maybe_download('notMNIST_large.tar.gz', 247336696)
test_filename = maybe_download('notMNIST_small.tar.gz', 8458043)

解决方案

I get everything, but the point where the function download_progress_hook gets called from within function maybe_download

That's where you went wrong. The function is not being called. It is only being referenced. There is no (...) call expression there.

Python functions are first-class objects, you can pass them around or assign them to other names:

>>> def foo(bar):
...     return bar + 1
...
>>> foo
<function foo at 0x100e20410>
>>> spam = foo
>>> spam
<function foo at 0x100e20410>
>>> spam(5)
6

Here spam is another reference to the function object foo. I can call that function object through that other name too.

So the following expression:

urlretrieve(
    url + filename, dest_filename,
    reporthook=download_progress_hook) 

doesn't call download_progress_hook. It merely gives that function object to the urlretrieve() function, and it is that code that'll call download_progress_hook somewhere (passing in the required arguments).

From the URLOpener.retrieve documentation (which ultimately handles that hook):

If reporthook is given, it must be a function accepting three numeric parameters: A chunk number, the maximum size chunks are read in and the total size of the download (-1 if unknown). It will be called once at the start and after each chunk of data is read from the network.

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