Constraints on Network Construction Using Python

Linux Ascend GPU CPU Model Development Beginner Intermediate Expert

Overview

MindSpore can compile user source code based on the Python syntax into computational graphs, and can convert common functions or instances inherited from nn.Cell into computational graphs. Currently, MindSpore does not support conversion of any Python source code into computational graphs. Therefore, there are constraints on source code compilation, including syntax constraints and network definition constraints. As MindSpore evolves, the constraints may change.

Syntax Constraints

Supported Python Data Types

Number: supports int, float, and bool. Complex numbers are not supported.
String
List: supports the append method only. Updating a list will generate a new list.
Tuple
Dictionary: The type of key should be String.

MindSpore Extended Data Type

Tensor: Tensor variables must be defined instances.

Expression Types

Operation	Description
Unary operator	`+`,`-`, and`not`. The operator `+` supports only scalars.
Binary operator	`+`, `-`, ``, `/`, `%`, `*` and `//`.
`if` expression	For example, `a = x if x < y else y`.
Comparison expression	`>`, `>=`, `<`, `<=`, `==`, and `! =`.
Logical expression	`and` and `or`.
`lambda` expression	For example, `lambda x, y: x + y`.
Reserved keyword type	`True`, `False`, and `None`.

Statement Types

Statement	Compared with Python
`def`	Same as that in Python.
`for`	Nested for loops are partially supported. Iteration sequences must be tuples or lists.
`while`	Nested while loops are partially supported. Grad of net with while is not supported.
`break`	Same as that in Python.
`if`	Same as that in Python. The input of the `if` condition must be a constant.
`in`	Only supports judging whether constants exist in Tuple/List/Dictionary whose elements are all constants.
`not in`	Only support Dictionary.
`is`	Only support `True`, `False`, and `None`.
`is not`	Only support `True`, `False`, and `None`.
Assignment statement	Accessed multiple subscripts of lists and dictionaries cannot be used as l-value.

System Functions/Classes

Functions/Class	Compared with Python
`len`	The usage principle is consistent with Python, and the returned result is consistent with Python, returning int.
`partial`	The usage principle is consistent with Python, and the returned result is inconsistent with Python, returning function.
`map`	The usage principle is consistent with Python, and the returned result is inconsistent with Python, returning tuple.
`zip`	The usage principle is consistent with Python, and the returned result is inconsistent with Python, returning tuple.
`range`	The usage principle is consistent with Python, and the returned result is inconsistent with Python, returning tuple.
`enumerate`	The usage principle is consistent with Python, and the returned result is inconsistent with Python, returning tuple.
`super`	The usage principle is consistent with Python, and the returned result is inconsistent with Python, returning the namespace defined by mindspore.
`isinstance`	The usage principle is consistent with Python, but the second input parameter can only be the type defined by mindspore.

Function Parameters

Default parameter value: The data types int, float, bool, None, str, tuple, list, and dict are supported, whereas Tensor is not supported.
Variable parameter: Functions with variable arguments is supported for training and inference.
Key-value pair parameter: Functions with key-value pair parameters cannot be used for backward propagation on computational graphs.
Variable key-value pair parameter: Functions with variable key-value pairs cannot be used for backward propagation on computational graphs.

Operators

Operator	Supported Type
`+`	Scalar, `Tensor`, `tuple` and `string`
`-`	Scalar and `Tensor`
`*`	Scalar and `Tensor`
`/`	Scalar and `Tensor`
`**`	Scalar and `Tensor`
`//`	Scalar and `Tensor`
`%`	Scalar and `Tensor`
`[]`	The operation object type can be `list`, `tuple`, or `Tensor`. Accessed multiple subscripts of lists and dictionaries can be used as r-values instead of l-values. Only when the operation object type is tuple or list with element type `nn.Cell`, the index type can be Tensor. For details about access constraints for the tuple and Tensor types, see the description of slicing operations.

Index operation

The index operation includes tuple and Tensor. The following focuses on the index value assignment and assignment operation of Tensor. The value takestensor_x [index]as an example, and the assignment takes tensor_x [index] = u as an example for detailed description. Among them, tensor_x is a Tensor, which is sliced; index means the index, u means the assigned value, which can be scalar or Tensor (size = 1). The index types are as follows:

Slice index: index is slice
- Value: tensor_x[start: stop: step], where Slice (start: stop: step) has the same syntax as Python, and will not be repeated here.
- Assignment: tensor_x[start: stop: step] = u.
Ellipsis index: index is ellipsis
- Value: tensor_x [...].
- Assignment: tensor_x [...] = u.
Boolean constant index: index is True, index is False is not supported temporarily.
- Value: tensor_x[True].
- Assignment: Not supported yet.
Tensor index: index is Tensor
- Value: tensor_x [index], index must be Tensor of data type int32 or int64, the element value range is [0, tensor_x.shape[0]).
- Assignment: tensor_x [index] = U.
  - tensor_x data type must be one of the following: float16, float32, int8, uint8.
  - index must be Tensor of data type int32, the element value range is [0, tensor_x.shape [0]).
  - U can be Number, Tensor, Tuple only containing Number, Tuple only containing Tensor.
    - Single Number or every Number in Tuple must be the same type as tensor_x, ie When the data type of tensor_x is uint8 or int8, the Number type should be int; When the data type of tensor_x is float16 or float32, the Number type should be float.
    - Single Tensor or every Tensor in Tuple must be consistent with the data type of tensor_x, when single Tensor, the shape should be equal to or broadcast as index.shape + tensor_x.shape [1:].
    - Tuple containing Number must meet requirement: len (Tuple) = (index.shape + tensor_x.shape [1:]) [-1].
    - Tuple containing Tensor must meet requirements: the shape of each Tensor should be the same, (len (Tuple),) + Tensor.shape should be equal to or broadcast as index.shape + tensor_x.shape [1:].
None constant index: index is None
- Value: tensor_x[None], results are consistent with numpy.
- Assignment: Not supported yet.
tuple index: index is tuple
- The tuple element is a slice:
  - Value: for example tensor_x[::,: 4, 3: 0: -1].
  - Assignment: for example tensor_x[::,: 4, 3: 0: -1] = u.
- The tuple element is Number:
  - Value: for example tensor_x[2,1].
  - Assignment: for example tensor_x[1,4] = u.
- The tuple element is a mixture of slice and ellipsis:
  - Value: for example tensor_x[..., ::, 1:].
  - Assignment: for example tensor_x[..., ::, 1:] = u.
- Not supported in other situations

The index value operation of tuple and list type, we need to focus on the index value operation of tuple or list whose element type is nn.Cell. This operation is currently only supported by the GPU backend in Graph mode, and its syntax format is like layers[index](*inputs), the example code is as follows:

class Net(nn.Cell):
    def __init__(self):
        super(Net, self).__init__()
        self.relu = nn.ReLU()
        self.softmax = nn.Softmax()
        self.layers = (self.relu, self.softmax)

    def construct(self, x, index):
        x = self.layers[index](x)
        return x

The grammar has the following constraints:

Only the index value operation of tuple or list whose element type is nn.Cell is supported.
The index is a scalar Tensor of type int32, with a value range of [-n, n), where n is the size of the tuple, and the maximum supported tuple size is 1000.
The number, type and shape of the input data of the Construct function of each Cell element in the tuple are the same, and the number of data output after the Construct function runs, the type and shape are also the same.
Each element in the tuple needs to be defined before the tuple is defined.

This syntax does not support running branches as if, while, for and other control flow, except if the control condition of the control flow is constant. for example:

Supported example:

class Net(nn.Cell):
  def __init__(self, flag=True):
      super(Net, self).__init__()
      self.flag = flag
      self.relu = nn.ReLU()
      self.softmax = nn.Softmax()
      self.layers = (self.relu, self.softmax)

  def construct(self, x, index):
      if self.flag:
        x = self.layers[index](x)
      return x

Unsupported example:

class Net(nn.Cell):
  def __init__(self):
      super(Net, self).__init__()
      self.relu = nn.ReLU()
      self.softmax = nn.Softmax()
      self.layers = (self.relu, self.softmax)

  def construct(self, x, index, flag):
      if flag:
        x = self.layers[index](x)
      return x

Tuple also support slice value operations, but do not support slice type as Tensor, support tuple_x [start: stop: step], which has the same effect as Python, and will not be repeated here.

Unsupported Syntax

Currently, the following syntax is not supported in network constructors: raise, yield, async for, with, async with, assert, import, and await.

Network Definition Constraints

Instance Types on the Entire Network

Common Python function with the @ms_function decorator.
Cell subclass inherited from nn.Cell.

Network Input Type

The training data input parameters of the entire network must be of the Tensor type.
The generated ANF diagram cannot contain the following constant nodes: string constants, constants with nested tuples, and constants with nested lists.

Network Graph Optimization

During graph optimization at the ME frontend, the dataclass, dictionary, list, and key-value pair types are converted to tuple types, and the corresponding operations are converted to tuple operations.

Network Construction Components

Category	Content
`Cell` instance	mindspore/nn/*, and custom Cell.
Member function of a `Cell` instance	Member functions of other classes in the construct function of Cell can be called.
Function	Custom Python functions and system functions listed in the preceding content.
Dataclass instance	Class decorated with @dataclass.
Primitive operator	mindspore/ops/operations/*.
Composite operator	mindspore/ops/composite/*.
Operator generated by constexpr	Uses the value generated by @constexpr to calculate operators.

Other Constraints

Input parameters of the construct function on the entire network and parameters of functions modified by the ms_function decorator are generalized during the graph compilation and cannot be passed to operators as constant input. Therefore, in graph mode, the parameter passed to the entry network can only be Tensor. As shown in the following example:
- The following is an example of incorrect input:
```
class ExpandDimsTest(Cell):
    def __init__(self):
        super(ExpandDimsTest, self).__init__()
        self.expandDims = ops.ExpandDims()

    def construct(self, input_x, input_axis):
        return self.expandDims(input_x, input_axis)
expand_dim = ExpandDimsTest()
input_x = Tensor(np.random.randn(2,2,2,2).astype(np.float32))
expand_dim(input_x, 0)
```
  In the example, ExpandDimsTest is a single-operator network with two inputs: input_x and input_axis. The second input of the ExpandDims operator must be a constant. This is because input_axis is required when the output dimension of the ExpandDims operator is deduced during graph compilation. As the network parameter input, the value of input_axis is generalized into a variable and cannot be determined. As a result, the output dimension of the operator cannot be deduced, causing the graph compilation failure. Therefore, the input required by deduction in the graph compilation phase must be a constant. In the API, the parameters of this type of operator that require constant input will be explained, marked const input is needed.
- Directly enter the needed value or a member variable in a class for the constant input of the operator in the construct function. The following is an example of correct input:
```
class ExpandDimsTest(Cell):
    def __init__(self, axis):
        super(ExpandDimsTest, self).__init__()
        self.expandDims = ops.ExpandDims()
        self.axis = axis

    def construct(self, input_x):
        return self.expandDims(input_x, self.axis)
axis = 0
expand_dim = ExpandDimsTest(axis)
input_x = Tensor(np.random.randn(2,2,2,2).astype(np.float32))
expand_dim(input_x)
```

It is not allowed to modify non-Parameter type data members of the network. Examples are as follows:

class Net(Cell):
    def __init__(self):
        super(Net, self).__init__()
        self.num = 2
        self.par = Parameter(Tensor(np.ones((2, 3, 4))), name="par")

    def construct(self, x, y):
        return x + y

In the network defined above, self.num is not a Parameter and cannot be modified, but self.par is a Parameter and can be modified.

When an undefined class member is used in the construct function, it will be treated as None instead of throwing AttributeError like the Python interpreter. Examples are as follows:
```
class Net(Cell):
    def __init__(self):
        super(Net, self).__init__()

    def construct(self, x):
        return x + self.y
```
In the network defined above, the undefined class member self.y is used in construct, and self.y will be treated as None.