Static Graph Syntax - Python Statements

Simple Statements

raise Statements

Support the use of raise to trigger an exception. raise syntax format: raise[Exception [, args]]. The Exception in the statement is the type of the exception, and the args is the user-supplied argument to the exception, usually a string or other object. The following types of errors are supported: NoExceptionType, UnknownError, ArgumentError, NotSupportError, NotExistsError, DeviceProcessError, AbortedError, IndexError, ValueError, TypeError, KeyError, AttributeError, NameError, AssertionError, BaseException, KeyboardInterrupt, Exception, StopIteration, OverflowError, ZeroDivisionError, EnvironmentError, IOError, OSError, ImportError, MemoryError, UnboundLocalError, RuntimeError, NotImplementedError, IndentationError, RuntimeWarning.

It is worth noting that the ability of the raise statement in the variable scenario in static graph mode is supported through extended syntax, that is, it needs to be supported only when the JIT syntax support level option ‘jit_syntax_level’ is ‘LAX’.

For example:

import mindspore.nn as nn
import mindspore as ms

class Net(nn.Cell):
    def __init__(self):
        super(Net, self).__init__()

    def construct(self, x, y):
        if x <= y:
            raise ValueError("x should be greater than y.")
        else:
            x += 1
        return x

ms.set_context(mode=ms.GRAPH_MODE)
net = Net()
net(ms.Tensor(-2), ms.Tensor(-1))

The output result:

ValueError: x should be greater than y.

assert Statements

Supports the use of assert for exception checking, assert syntax format: assert[Expression [, args]], where Expression is the judgment condition. If the condition is true, nothing will be done, while if the condition is false, an exception message of type AssertError will be thrown. The args are user-supplied exception arguments, which can usually be strings or other objects.

import mindspore.nn as nn
import mindspore as ms

class Net(nn.Cell):
    def __init__(self):
        super(Net, self).__init__()

    def construct(self, x):
        assert x in [2, 3, 4]
        return x

ms.set_context(mode=ms.GRAPH_MODE)
net = Net()
net(ms.Tensor(-1))

Appears normally in the output:

AssertionError.

pass Statements

The pass statement doesn’t do anything and is usually used as a placeholder to maintain structural integrity. For example:

import mindspore as ms
from mindspore import nn, set_context

set_context(mode=ms.GRAPH_MODE)

class Net(nn.Cell):
  def construct(self, x):
    i = 0
    while i < 5:
      if i > 3:
        pass
      else:
        x = x * 1.5
      i += 1
    return x

net = Net()
ret = net(10)
print("ret:", ret)

The result is as follows:

ret: 50.625

return Statements

The return statement usually returns the result to the place where it was called, and statements after the return statement are not executed. If the return statement does not have any expression or the function does not have a return statement, a None object is returned by default. There can be more than one return statement within a function, depending on the situation. For example:

import mindspore as ms
from mindspore import nn, set_context

set_context(mode=ms.GRAPH_MODE)

class Net(nn.Cell):
  def construct(self, x):
      if x > 0:
        return x
      else:
        return 0

net = Net()
ret = net(10)
print("ret:", ret)

The result is as follows:

ret: 10

As above, there can be multiple return statements in a control flow scenario statement. If there is no return statement in a function, the None object is returned by default, as in the following use case:

from mindspore import jit, context

context.set_context(mode=context.GRAPH_MODE)

@jit
def foo():
  x = 3
  print("x:", x)

res = foo()
assert res is None

break Statements

The break statement is used to terminate a loop statement, i.e., it stops execution of the loop statement even if the loop condition does not have a False condition or if the sequence is not fully recursive, usually used in while and for loops. In nested loops, the break statement stops execution of the innermost loop.

import mindspore as ms
from mindspore import nn, set_context

set_context(mode=ms.GRAPH_MODE)

class Net(nn.Cell):
  def construct(self, x):
    for i in range(8):
      if i > 5:
        x *= 3
        break
      x = x * 2
    return x

net = Net()
ret = net(10)
print("ret:", ret)

The result is as follows:

ret: 1920

continue Statements

The continue statement is used to jump out of the current loop statement and into the next round of the loop. This is different from the break statement, which is used to terminate the entire loop statement. continue is also used in while and for loops. For example:

import mindspore as ms
from mindspore import nn, set_context

set_context(mode=ms.GRAPH_MODE)

class Net(nn.Cell):
  def construct(self, x):
    for i in range(4):
      if i > 2:
        x *= 3
        continue
    return x


net = Net()
ret = net(3)
print("ret:", ret)

The result is as follows:

ret: 9

Compound Statements

Conditional Control Statements

if Statements

Usage:

if (cond): statements...
x = y if (cond) else z

Parameter: cond – Variables of bool type and constants of bool, list, tuple, dict and string types are supported.

Restrictions:

If cond is not a constant, the variable or constant assigned to a same sign in different branches should have same data type. If the data type of assigned variables or constants is Tensor, the variables and constants should have same shape and element type.

Example 1:

import mindspore as ms

x = ms.Tensor([1, 4], ms.int32)
y = ms.Tensor([0, 3], ms.int32)
m = 1
n = 2

@ms.jit()
def test_cond(x, y):
    if (x > y).any():
        return m
    else:
        return n

ret = test_cond(x, y)
print('ret:{}'.format(ret))

The data type of m returned by the if branch and n returned by the else branch must be same.

The result is as follows:

ret:1

Example 2:

import mindspore as ms

x = ms.Tensor([1, 4], ms.int32)
y = ms.Tensor([0, 3], ms.int32)
m = 1
n = 2

@ms.jit()
def test_cond(x, y):
    out = 3
    if (x > y).any():
        out = m
    else:
        out = n
    return out

ret = test_cond(x, y)
print('ret:{}'.format(ret))

The variable or constant m assigned to out in if branch and the variable or constant n assigned to out in false branch must have same data type.

The result is as follows:

ret:1

Example 3:

import mindspore as ms

x = ms.Tensor([1, 4], ms.int32)
y = ms.Tensor([0, 3], ms.int32)
m = 1

@ms.jit()
def test_cond(x, y):
    out = 2
    if (x > y).any():
        out = m
    return out

ret = test_cond(x, y)
print('ret:{}'.format(ret))

The variable or constant m assigned to out in if branch and the variable or constant init initially assigned to out must have same data type.

The result is as follows:

ret:1

Loop Statements

for Statements

Usage:

for i in sequence statements...
for i in sequence statements... if (cond) break
for i in sequence statements... if (cond) continue

Parameter: sequence – Iterative sequences (Tuple, List, range and so on).

Restrictions:

The total number of graph operations is a multiple of number of iterations of the for loop. Excessive number of iterations of the for loop may cause the graph to occupy more memory than usage limit.
The for...else... statement is not supported.

Example:

import numpy as np
import mindspore as ms

z = ms.Tensor(np.ones((2, 3)))

@ms.jit()
def test_cond():
    x = (1, 2, 3)
    for i in x:
        z += i
    return z

ret = test_cond()
print('ret:{}'.format(ret))

The result is as follows:

ret:[[7. 7. 7.]
 [7. 7. 7.]]

while Statements

Usage:

while (cond) statements...
while (cond) statements... if (cond1) break
while (cond) statements... if (cond1) continue

Parameter: cond – Variables of bool type and constants of bool, list, tuple, dict and string types are supported.

Restrictions:

If cond is not a constant, the variable or constant assigned to a same sign inside body of while and outside body of while should have same data type.If the data type of assigned variables or constants is Tensor, the variables and constants should have same shape and element type.
The while...else... statement is not supported.

Example 1:

import mindspore as ms

m = 1
n = 2

@ms.jit()
def test_cond(x, y):
    while x < y:
        x += 1
        return m
    return n

ret = test_cond(1, 5)
print('ret:{}'.format(ret))

The data type of m returned inside while and data type of n returned outside while must have same data type.

The result is as follows:

ret:1

Example 2:

import mindspore as ms

m = 1
n = 2

def ops1(a, b):
    return a + b

@ms.jit()
def test_cond(x, y):
    out = m
    while x < y:
        x += 1
        out = ops1(out, x)
    return out

ret = test_cond(1, 5)
print('ret:{}'.format(ret))

The variable op1 assigned to out inside while and the variable or constant init initially assigned to out must have same data type.

The result is as follows:

ret:15

Function Definition Statements

def Keyword

def is used to define a function, followed by the function identifier name and the original parentheses (), which may contain the function parameters. Usage: def function_name(args): statements....

For example:

import mindspore as ms

def number_add(x, y):
    return x + y

@ms.jit()
def test(x, y):
    return number_add(x, y)

ret = test(1, 5)
print('ret:{}'.format(ret))

The result is as follows:

ret:6

Instructions:

The defined function supported has no return statement. That means the return value of default functions is None.
Construct function of the outermost network and the inner network function is support kwargs, like:def construct(**kwargs):.
Mixed use of variable argument and non-variable argument is supported, like:def function(x, y, *args) and def function(x = 1, y = 1, **kwargs).

lambda Expression

A lambda expression is used to generate an anonymous function. Unlike normal functions, it computes and returns only one expression. Usage: lambda x, y: x + y.

For example:

import mindspore as ms

@ms.jit()
def test(x, y):
    number_add = lambda x, y: x + y
    return number_add(x, y)

ret = test(1, 5)
print('ret:{}'.format(ret))

The result is as follows:

ret:6

Partial function partial

Function: partial function, fixed function input parameter. Usage: partial(func, arg, ...).

Input parameter:

func – function.
arg – One or more parameters to be fixed, support positional parameters and key-value pair parameters.

Return Value: Returns some functions with fixed input value.

The example is as follows:

import mindspore as ms
from mindspore import ops

def add(x, y):
    return x + y

@ms.jit()
def test():
    add_ = ops.partial(add, x=2)
    m = add_(y=3)
    n = add_(y=5)
    return m, n

m, n = test()
print('m:{}'.format(m))
print('n:{}'.format(n))

The result is as follows:

m:5
n:7

Function Parameters

Default parameter value: The default value set to Tensor type data is currently not supported, and int, float, bool, None, str, tuple, list, dict type data is supported.
Variable parameters: Inference and training of networks with variable parameters are supported.
Key-value pair parameter: Functions with key-value pair parameters cannot be used for backward propagation.
Variable key-value pair parameter: Functions with variable key-value pairs cannot be used for backward propagation.

List Comprehension and Generator Expression

Support for List Comprehension and Generator Expression. Support for constructing a new sequence.

List Comprehension

List comprehension are used to generate lists. Usage: [arg for loop if statements].

The example is as follows:

import mindspore as ms

@ms.jit()
def test():
    l = [x * x for x in range(1, 11) if x % 2 == 0]
    return l

ret = test()
print('ret:{}'.format(ret))

The result is as follows:

ret:[4, 16, 36, 64, 100]

Restrictions:

The use of multiple levels of nested iterators is not supported in graph mode.

The example usage of the restriction is as follows (two levels of iterators are used):

l = [y for x in ((1, 2), (3, 4), (5, 6)) for y in x]

An error will be prompted:

TypeError:  The `generators` supports one `comprehension` in ListComp/GeneratorExp, but got 2 comprehensions.

Dict Comprehension

Dict comprehension is used to generate lists. Usage: {key, value for loop if statements}.

The example is as follows:

import mindspore as ms

@ms.jit()
def test():
    x = [('a', 1), ('b', 2), ('c', 3)]
    res = {k: v for (k, v) in x if v > 1}
    return res

ret = test()
print('ret:{}'.format(ret))

The result is as follows:

ret:{'b': 2, 'c': 3}

Restrictions:

The use of multi-layer nested iterators is not supported in graph mode.

The example usage of the restriction is as follows (two levels of iterators are used):

x = ({'a': 1, 'b': 2}, {'d': 1, 'e': 2}, {'g': 1, 'h': 2})
res = {k: v for y in x for (k, v) in y.items()}

An error will be prompted:

TypeError:  The `generators` supports one `comprehension` in DictComp/GeneratorExp, but got 2 comprehensions.

Generator Expression

Generator expressions are used to generate lists. Usage: (arg for loop if statements).

For example:

import mindspore as ms

@ms.jit()
def test():
    l = (x * x for x in range(1, 11) if x % 2 == 0)
    return l

ret = test()
print('ret:{}'.format(ret))

The result is as follows:

ret:[4, 16, 36, 64, 100]

Usage restrictions are the same as list comprehension, i.e., the use of multiple levels of nested iterators is not supported in graph mode.

With Statement

In graph mode, the with statement is supported with limitations. The with statement requires that the object must have two magic methods: __enter__() and __exit__().

It is worth noting that the class used in the with statement needs to be decorated with a decorator@ms.jit_class or inherited from nn. Cell, more on this can be found in Calling the Custom Class

For example:

import mindspore as ms
import mindspore.nn as nn
from mindspore import set_context

set_context(mode=ms.GRAPH_MODE)

@ms.jit_class
class Sample:
    def __init__(self):
        super(Sample, self).__init__()
        self.num = ms.Tensor([2])

    def __enter__(self):
        return self.num * 2

    def __exit__(self, exc_type, exc_value, traceback):
        return self.num * 4

class TestNet(nn.Cell):
    def construct(self):
        res = 1
        obj = Sample()
        with obj as sample:
            res += sample
        return res, obj.num

test_net = TestNet()
out1, out2 = test_net()
print("out1:", out1)
print("out2:", out2)

The result is as follows:

out1: [5]
out2: [2]