Pytorch: parameters(),children(),modules(),named_*区别

nn.Module vs nn.functional

前者会保存权重等信息，后者只是做运算

parameters()

返回可训练参数

nn.ModuleList vs. nn.ParameterList vs. nn.Sequential

layer_list = [nn.Conv2d(5,5,3), nn.BatchNorm2d(5), nn.Linear(5,2)]
class myNet(nn.Module):
  def __init__(self):
    super().__init__()
    self.layers = layer_list
  def forward(x):
    for layer in self.layers:
      x = layer(x)
net = myNet()
print(list(net.parameters()))  # Parameters of modules in the layer_list don't show up.

nn.ModuleList的作用就是wrap pthon list，这样其中的参数会被注册，因此可以返回可训练参数(ParameterList)。

nn.Sequential的作用如下：

class myNet(nn.Module):
  def __init__(self):
    super().__init__()
    self.layers = nn.Sequential(
        nn.Relu(inplace=True),
        nn.Linear(10, 10)
    )
  def forward(x):
    x = layer(x)
x = torch.rand(10)
net = myNet()
print(net(x).shape)

可以看到Sequential的作用就是按照指定的顺序构建网络结构，得到一个完整的模块，而ModuleList则只是像list那样把元素集合起来而已。

nn.modules vs. nn.children

class myNet(nn.Module):
  def __init__(self):
    super().__init__()
    self.convBN =  nn.Sequential(nn.Conv2d(10,10,3), nn.BatchNorm2d(10))
    self.linear =  nn.Linear(10,2)
  def forward(self, x):
    pass
Net = myNet()
print("Printing children\n------------------------------")
print(list(Net.children()))
print("\n\nPrinting Modules\n------------------------------")
print(list(Net.modules()))

输出信息如下：

Printing children
------------------------------
[Sequential(
  (0): Conv2d(10, 10, kernel_size=(3, 3), stride=(1, 1))
  (1): BatchNorm2d(10, eps=1e-05, momentum=0.1, affine=True, track_running_stats=True)
), Linear(in_features=10, out_features=2, bias=True)]
Printing Modules
------------------------------
[myNet(
  (convBN1): Sequential(
    (0): Conv2d(10, 10, kernel_size=(3, 3), stride=(1, 1))
    (1): BatchNorm2d(10, eps=1e-05, momentum=0.1, affine=True, track_running_stats=True)
  )
  (linear): Linear(in_features=10, out_features=2, bias=True)
), Sequential(
  (0): Conv2d(10, 10, kernel_size=(3, 3), stride=(1, 1))
  (1): BatchNorm2d(10, eps=1e-05, momentum=0.1, affine=True, track_running_stats=True)
), Conv2d(10, 10, kernel_size=(3, 3), stride=(1, 1)), BatchNorm2d(10, eps=1e-05, momentum=0.1, affine=True, track_running_stats=True), Linear(in_features=10, out_features=2, bias=True)]

可以看到children只会返回子元素，子元素可能是单个操作，如Linear，也可能是Sequential。 而modules()返回的信息更加详细，不仅会返回children一样的信息，同时还会递归地返回，例如modules()会迭代地返回Sequential中包含的若干个子元素。

named_*

• named_parameters: 返回一个iterator,每次它会提供包含参数名的元组。

In [27]: x = torch.nn.Linear(2,3)
In [28]: x_name_params = x.named_parameters()
In [29]: next(x_name_params)
Out[29]:
('weight', Parameter containing:
 tensor([[-0.5262,  0.3480],
         [-0.6416, -0.1956],
         [ 0.5042,  0.6732]], requires_grad=True))
In [30]: next(x_name_params)
Out[30]:
('bias', Parameter containing:
 tensor([ 0.0595, -0.0386,  0.0975], requires_grad=True))

• named_modules
  
  这个其实就是把上面提到的nn.modules以iterator的形式返回，每次读取和上面一样也是用next()，示例如下：

In [46]:  class myNet(nn.Module):
    ...:    def __init__(self):
    ...:      super().__init__()
    ...:      self.convBN1 =  nn.Sequential(nn.Conv2d(10,10,3), nn.BatchNorm2d(10))
    ...:      self.linear =  nn.Linear(10,2)
    ...:
    ...:    def forward(self, x):
    ...:      pass
    ...:                                                                                   
In [47]: net = myNet()                                                                     
In [48]: net_named_modules = net.named_modules()                                           
In [49]: next(net_named_modules)
Out[49]:
('', myNet(
   (convBN1): Sequential(
     (0): Conv2d(10, 10, kernel_size=(3, 3), stride=(1, 1))
     (1): BatchNorm2d(10, eps=1e-05, momentum=0.1, affine=True, track_running_stats=True)
   )
   (linear): Linear(in_features=10, out_features=2, bias=True)
 ))                                                                                        
In [50]: next(net_named_modules)
Out[50]:
('convBN1', Sequential(
   (0): Conv2d(10, 10, kernel_size=(3, 3), stride=(1, 1))
   (1): BatchNorm2d(10, eps=1e-05, momentum=0.1, affine=True, track_running_stats=True)
 ))                                                                                        
In [51]: next(net_named_modules)
Out[51]: ('convBN1.0', Conv2d(10, 10, kernel_size=(3, 3), stride=(1, 1)))                  
In [52]: next(net_named_modules)
Out[52]:
('convBN1.1',
 BatchNorm2d(10, eps=1e-05, momentum=0.1, affine=True, track_running_stats=True))          
In [53]: next(net_named_modules)
Out[53]: ('linear', Linear(in_features=10, out_features=2, bias=True))                     
In [54]: next(net_named_modules)
---------------------------------------------------------------------------
StopIteration                             Traceback (most recent call last)
<ipython-input-54-05e848b071b8> in <module>
----> 1 next(net_named_modules)
StopIteration:

• named_children

同named_modules

参考

https://blog.paperspace.com/pytorch-101-advanced/