# 报错ValueError: For xxx,x和y的shape不支持 broadcast

**1. 报错描述**

1.1 系统环境

```
Hardware Environment(Ascend/GPU/CPU): Ascend Software Environment: -- MindSpore version (source or binary): 1.6.0 -- Python version (e.g., Python 3.7.5): 3.7.6 -- OS platform and distribution (e.g., Linux Ubuntu 16.04): Ubuntu 4.15.0-74-generic -- GCC/Compiler version (if compiled from source):
```

1.2 基本信息

1.2.1 脚本

训练脚本是通过构建Abs的单算子网络，对输入两个张量做Sub运算后再计算Abs。脚本如下：

```
 class Net(nn.Cell):
   def __init__(self):
     super(Net, self).__init__()
     self.abs = ops.Abs()

   def construct(self, x1,x2):
     output = self.abs(x1 - x2)
     return output
 net = Net()
 x1 = Tensor(np.ones((2, 5), dtype=np.float32), mindspore.float32)
 x2 = Tensor(np.ones((3, 5), dtype=np.float32), mindspore.float32)
 out = net(x1,x2)
 print('out',out.shape)
```

**2. 报错**

这里报错信息如下：

```
The function call stack (See file '/demo/rank_0/om/analyze_fail.dat' for more details):
# 0 In file demo.py(7)
         output = self.abs(x1 - x2)
                           ^

Traceback (most recent call last):
  File "demo.py", line 13, in <module>
    out = net(x1,x2)
  File "/lib/python3.7/site-packages/mindspore/nn/cell.py", line 576, in __call__
    out = self.compile_and_run(*args)
  File "/lib/python3.7/site-packages/mindspore/nn/cell.py", line 942, in compile_and_run
    self.compile(*inputs)
  File "/lib/python3.7/site-packages/mindspore/nn/cell.py", line 915, in compile
    _cell_graph_executor.compile(self, *inputs, phase=self.phase, auto_parallel_mode=self._auto_parallel_mode)
  File "/lib/python3.7/site-packages/mindspore/common/api.py", line 791, in compile
    result = self._graph_executor.compile(obj, args_list, phase, self._use_vm_mode())
  File "/lib/python3.7/site-packages/mindspore/ops/primitive.py", line 575, in __infer__
    out[track] = fn(*(x[track] for x in args))
  File "/lib/python3.7/site-packages/mindspore/ops/operations/math_ops.py", line 78, in infer_shape
    return get_broadcast_shape(x_shape, y_shape, self.name)
  File "/lib/python3.7/site-packages/mindspore/ops/_utils/utils.py", line 70, in get_broadcast_shape
    raise ValueError(f"For '{prim_name}', {arg_name1}.shape and {arg_name2}.shape are supposed "
ValueError: For 'Sub', x.shape and y.shape are supposed to broadcast, where broadcast means that x.shape[i] = 1 or -1 or y.shape[i] = 1 or -1 or x.shape[i] = y.shape[i], but now x.shape and y.shape can not broadcast, got i: -2, x.shape: [2, 5], y.shape: [3, 5].
```

原因分析

我们看报错信息，在ValueError中，写到ValueError: For 'Sub', x.shape and y.shape are supposed to broadcast, where broadcast means that x.shape = 1 or -1 or y.shape = 1 or -1 or x.shape = y.shape，意思是abs的两个操作对象不能进行broadcast，broadcast的要求是x.shape = 1 or -1 or y.shape = 1 or -1 or x.shape = y.shape，而x.shape = y.shape要求两个参数的shape完全相等，在进一步的报错信息中也有写到but now x.shape and y.shape can not broadcast, got i: -2, x.shape: \[2, 5\], y.shape: \[3, 5\]，显然，x和y的第一个维度不等，这就是问题出现的原因了。关于BroadCast，在官网做了输入限制，对输入的Tensor要求shape必须相同。在其他的双输入算子中，有一定量算子用到了BroadCast操作，也应当注意这点。

**3. 解决方法**

基于上面已知的原因，很容易做出如下修改： 示例1：

![](figures/11-70.png)

此时执行成功，输出如下：

```
out: (3, 5)
```

示例2：

```
class Net(nn.Cell):
   def __init__(self):
​     super(Net, self).__init__()
​     self.abs = ops.Abs()

   def construct(self, x1,x2):
​     output = self.abs(x1 - x2)
​     return output
 net = Net()
 x1 = Tensor(np.ones((5), dtype=np.float32), mindspore.float32)
 x2 = Tensor(np.ones((3, 5), dtype=np.float32), mindspore.float32)
 out = net(x1,x2)
 print('out',out.shape)
```

此时执行成功，输出如下：

```
out: (3, 5)
```

**4. 总结**

定位报错问题的步骤：

1、找到报错的用户代码行：output = self.abs\(x1 - x2\);

2、 根据日志报错信息中的关键字，缩小分析问题的范围：x.shape: \[2, 5\], y.shape: \[3, 5\];

3、需要重点关注变量定义、初始化的正确性。

**5. 参考文档**

5.1  [broadcast方法](https://www.mindspore.cn/docs/api/zh-CN/r1.5/api_python/ops/mindspore.ops.Broadcast.html?highlight=broadcast)

