Colon in python class body

Question

Analizing Monai AI documentation found following code, I've never seen colon operator used in this way. How to interpret this: mod: nn.Module?

def _get_down_layer(self, in_channels: int, out_channels: int, strides: int, is_top: bool) -> nn.Module:
        """
        Args:
            in_channels: number of input channels.
            out_channels: number of output channels.
            strides: convolution stride.
            is_top: True if this is the top block.
        """
        mod: nn.Module
        if self.num_res_units > 0:

            mod = ResidualUnit(
                self.dimensions,
                in_channels,
                out_channels,
                strides=strides,
                kernel_size=self.kernel_size,
                subunits=self.num_res_units,
                act=self.act,
                norm=self.norm,
                dropout=self.dropout,
                bias=self.bias,
            )
            return mod
        mod = Convolution(
            self.dimensions,
            in_channels,
            out_channels,
            strides=strides,
            kernel_size=self.kernel_size,
            act=self.act,
            norm=self.norm,
            dropout=self.dropout,
            bias=self.bias,
        )
        return mod

reference to whole monai documentation

Answer 1

The usage of variable_name: variable_type is used to 'hint' the type of a variable. It doesn't set or modify a variable. I just hints what kind of variable it is.

It doesn't have any functionality but to help others understand the code.