Type Checks

In this section, you will learn about the following things:

  • Basic usage of type check

  • Detail of type information

  • Internal mechanism of type check

  • More complicated cases

  • Call functions

  • Typical type check example

After reading this section, you will be able to:

  • Write a code to check types of input arguments of your own functions

Basic usage of type check

When you call a function with an invalid type of array, you sometimes receive no error, but get an unexpected result by broadcasting. When you use CUDA with an illegal type of array, it causes memory corruption, and you get a serious error. These bugs are hard to fix. Chainer can check preconditions of each function, and helps to prevent such problems. These conditions may help a user to understand specification of functions.

Each implementation of Function has a method for type check, check_type_forward(). This function is called just before the forward() method of the Function class. You can override this method to check the condition on types and shapes of arguments.

check_type_forward() gets an argument in_types:

def check_type_forward(self, in_types):

in_types is an instance of TypeInfoTuple, which is a sub-class of tuple. To get type information about the first argument, use in_types[0]. If the function gets multiple arguments, we recommend to use new variables for readability:

x_type, y_type = in_types

In this case, x_type represents the type of the first argument, and y_type represents the second one.

We describe usage of in_types with an example. When you want to check if the number of dimension of x_type equals to 2, write this code:

utils.type_check.expect(x_type.ndim == 2)

When this condition is true, nothing happens. Otherwise this code throws an exception, and the user gets a message like this:

Traceback (most recent call last):
chainer.utils.type_check.InvalidType: Expect: in_types[0].ndim == 2
Actual: 3 != 2

This error message means that “ndim of the first argument expected to be 2, but actually it is 3”.

Detail of type information

You can access three information of x_type.

  • .shape is a tuple of ints. Each value is size of each dimension.

  • .ndim is int value representing the number of dimensions. Note that ndim == len(shape)

  • .dtype is numpy.dtype representing data type of the value.

You can check all members. For example, the size of the first dimension must be positive, you can write like this:

utils.type_check.expect(x_type.shape[0] > 0)

You can also check data types with .dtype:

utils.type_check.expect(x_type.dtype == np.float64)

And an error is like this:

Traceback (most recent call last):
chainer.utils.type_check.InvalidType: Expect: in_types[0].dtype == <class 'numpy.float64'>
Actual: float32 != <class 'numpy.float64'>

You can also check kind of dtype. This code checks if the type is floating point

utils.type_check.expect(x_type.dtype.kind == 'f')

You can compare between variables. For example, the following code checks if the first argument and the second argument have the same length:

utils.type_check.expect(x_type.shape[1] == y_type.shape[1])

Internal mechanism of type check

How does it show an error message like "in_types[0].ndim == 2"? If x_type is an object containing ndim member variable, we cannot show such an error message because this equation is evaluated as a boolean value by Python interpreter.

Actually x_type is a Expr objects, and doesn’t have a ndim member variable itself. Expr represents a syntax tree. x_type.ndim makes a Expr object representing (getattr, x_type, 'ndim'). x_type.ndim == 2 makes an object like (eq, (getattr, x_type, 'ndim'), 2). expect() gets a Expr object and evaluates it. When it is True, it causes no error and shows nothing. Otherwise, this method shows a readable error message.

If you want to evaluate a Expr object, call eval() method:

actual_type = x_type.eval()

actual_type is an instance of TypeInfo, while x_type is an instance of Expr. In the same way, x_type.shape[0].eval() returns an int value.

More powerful methods

Expr class is more powerful. It supports all mathematical operators such as + and *. You can write a condition that the first dimension of x_type is the first dimension of y_type times four:

utils.type_check.expect(x_type.shape[0] == y_type.shape[0] * 4)

When x_type.shape[0] == 3 and y_type.shape[0] == 1, users can get the error message below:

Traceback (most recent call last):
chainer.utils.type_check.InvalidType: Expect: in_types[0].shape[0] == in_types[1].shape[0] * 4
Actual: 3 != 4

To compare a member variable of your function, wrap a value with Variable to show readable error message:

x_type.shape[0] == utils.type_check.Variable(self.in_size, "in_size")

This code can check the equivalent condition below:

x_type.shape[0] == self.in_size

However, the latter condition doesn’t know the meaning of this value. When this condition is not satisfied, the latter code shows unreadable error message:

chainer.utils.type_check.InvalidType: Expect: in_types[0].shape[0] == 4  # what does '4' mean?
Actual: 3 != 4

Note that the second argument of utils.type_check.Variable is only for readability.

The former shows this message:

chainer.utils.type_check.InvalidType: Expect: in_types[0].shape[0] == in_size  # OK, `in_size` is a value that is given to the constructor
Actual: 3 != 4  # You can also check actual value here

Call functions

How to check summation of all values of shape? Expr also supports function call:

sum = utils.type_check.Variable(np.sum, 'sum')
utils.type_check.expect(sum(x_type.shape) == 10)

Why do we need to wrap the function numpy.sum with utils.type_check.Variable? x_type.shape is not a tuple but an object of Expr as we have seen before. Therefore, numpy.sum(x_type.shape) fails. We need to evaluate this function lazily.

The above example produces an error message like this:

Traceback (most recent call last):
chainer.utils.type_check.InvalidType: Expect: sum(in_types[0].shape) == 10
Actual: 7 != 10

More complicated cases

How to write a more complicated condition that can’t be written with these operators? You can evaluate Expr and get its result value with eval() method. Then check the condition and show warning message by hand:

x_shape = x_type.shape.eval()  # get actual shape (int tuple)
if not more_complicated_condition(x_shape):
    expect_msg = 'Shape is expected to be ...'
    actual_msg = 'Shape is ...'
    raise utils.type_check.InvalidType(expect_msg, actual_msg)

Please write a readable error message. This code generates the following error message:

Traceback (most recent call last):
chainer.utils.type_check.InvalidType: Expect: Shape is expected to be ...
Actual: Shape is ...

Typical type check example

We show a typical type check for a function.

First check the number of arguments:

utils.type_check.expect(in_types.size() == 2)

in_types.size() returns a Expr object representing the number of arguments. You can check it in the same way.

And then, get each type:

x_type, y_type = in_types

Don’t get each value before checking in_types.size(). When the number of argument is illegal, type_check.expect might output unuseful error messages. For example, this code doesn’t work when the size of in_types is 0:

  in_types.size() == 2,
  in_types[0].ndim == 3,

After that, check each type:

  x_type.dtype == np.float32,
  x_type.ndim == 3,
  x_type.shape[1] == 2,

The above example works correctly even when x_type.ndim == 0 as all conditions are evaluated lazily.