torch.nn.utils.rnn.pad_sequence(sequences, batch_first=False, padding_value=0.0)[source]

Pad a list of variable length Tensors with padding_value.

pad_sequence stacks a list of Tensors along a new dimension, and pads them to equal length. For example, if the input is a list of sequences with size L x * and batch_first is False, the output is of size T x B x *.

B is batch size. It is equal to the number of elements in sequences. T is length of the longest sequence. L is length of the sequence. * is any number of trailing dimensions, including none.


>>> from torch.nn.utils.rnn import pad_sequence
>>> a = torch.ones(25, 300)
>>> b = torch.ones(22, 300)
>>> c = torch.ones(15, 300)
>>> pad_sequence([a, b, c]).size()
torch.Size([25, 3, 300])


This function returns a Tensor of size T x B x * or B x T x * where T is the length of the longest sequence. This function assumes trailing dimensions and type of all the Tensors in sequences are same.

  • sequences (list[Tensor]) – list of variable length sequences.

  • batch_first (bool, optional) – output will be in B x T x * if True, or in T x B x * otherwise. Default: False.

  • padding_value (float, optional) – value for padded elements. Default: 0.


Tensor of size T x B x * if batch_first is False. Tensor of size B x T x * otherwise

Return type



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