{"record":{"id":"9ca6d8dcebb544f3","repo":"d2l-ai/d2l-zh","slug":"notimplementederror-9ca6d8","errorCode":null,"errorMessage":"NotImplementedError","messagePattern":"NotImplementedError","errorType":"exception","errorClass":"NotImplementedError","httpStatus":null,"severity":"error","filePath":"d2l/torch.py","lineNumber":936,"sourceCode":"    text = preprocess_nmt(read_data_nmt())\n    source, target = tokenize_nmt(text, num_examples)\n    src_vocab = d2l.Vocab(source, min_freq=2,\n                          reserved_tokens=['<pad>', '<bos>', '<eos>'])\n    tgt_vocab = d2l.Vocab(target, min_freq=2,\n                          reserved_tokens=['<pad>', '<bos>', '<eos>'])\n    src_array, src_valid_len = build_array_nmt(source, src_vocab, num_steps)\n    tgt_array, tgt_valid_len = build_array_nmt(target, tgt_vocab, num_steps)\n    data_arrays = (src_array, src_valid_len, tgt_array, tgt_valid_len)\n    data_iter = d2l.load_array(data_arrays, batch_size)\n    return data_iter, src_vocab, tgt_vocab\n\nclass Encoder(nn.Module):\n    \"\"\"编码器-解码器架构的基本编码器接口\"\"\"\n    def __init__(self, **kwargs):\n        super(Encoder, self).__init__(**kwargs)\n\n    def forward(self, X, *args):\n        raise NotImplementedError\n\nclass Decoder(nn.Module):\n    \"\"\"编码器-解码器架构的基本解码器接口\n\n    Defined in :numref:`sec_encoder-decoder`\"\"\"\n    def __init__(self, **kwargs):\n        super(Decoder, self).__init__(**kwargs)\n\n    def init_state(self, enc_outputs, *args):\n        raise NotImplementedError\n\n    def forward(self, X, state):\n        raise NotImplementedError\n\nclass EncoderDecoder(nn.Module):\n    \"\"\"编码器-解码器架构的基类\n\n    Defined in :numref:`sec_encoder-decoder`\"\"\"","sourceCodeStart":918,"sourceCodeEnd":954,"githubUrl":"https://github.com/d2l-ai/d2l-zh/blob/e6b18ccea71451a55fcd861d7b96fddf2587b09a/d2l/torch.py#L918-L954","documentation":"NotImplementedError raised by Encoder.forward in d2l.torch (the abstract encoder interface for the encoder-decoder architecture, sec_encoder-decoder). Encoder subclasses nn.Module but provides no default forward behavior; instantiating it directly and calling it (or using a subclass that fails to override forward) hits the guard. Concrete encoders like Seq2SeqEncoder supply the real forward.","triggerScenarios":"e = d2l.Encoder(); e(X); a subclass whose forward is misspelled (e.g. forwad) or defined outside the class body because of indentation, so nn.Module.__call__ falls through to the base method; passing a bare Encoder as the encoder member of EncoderDecoder and running a forward pass.","commonSituations":"D2L readers probing the interface; refactoring that swaps in the base class during tests; copy-paste from mxnet-flavored code where the subclass was written for nn.Block; IDE auto-import pulling d2l.Encoder instead of the concrete class.","solutions":["Instantiate a concrete encoder: d2l.Seq2SeqEncoder(vocab_size, embed_size, num_hiddens, num_layers)","In your subclass, define forward(self, X, *args) exactly (check spelling) at class-body indentation","Smoke-test immediately after construction: model(enc_X, dec_X) on one batch to surface missing overrides early","Lint with a check that MyEncoder.__dict__ contains 'forward' before use"],"exampleFix":"# before\nenc = d2l.Encoder()\nenc(X)  # NotImplementedError\n# after\nenc = d2l.Seq2SeqEncoder(vocab_size=10, embed_size=8, num_hiddens=16, num_layers=2)\nenc(X)  # (output, state)\n# or subclass\nclass MyEncoder(d2l.Encoder):\n    def forward(self, X, *args):\n        return X, None","handlingStrategy":"type-guard","validationCode":"import d2l\nassert type(encoder) is not d2l.Encoder and type(encoder).forward is not d2l.Encoder.forward, \\\n    'use a concrete Encoder subclass (e.g. Seq2SeqEncoder)'","typeGuard":"def is_concrete_encoder(obj) -> bool:\n    return isinstance(obj, d2l.Encoder) and type(obj).forward is not d2l.Encoder.forward","tryCatchPattern":null,"preventionTips":["Never call d2l.Encoder() directly","Grep subclass body for 'def forward' after writing it","Run encoder(X) on one batch immediately after construction","Import concrete classes (Seq2SeqEncoder) rather than the interface"],"tags":["d2l","pytorch","not-implemented","abstract-class","encoder-decoder","inheritance"],"backgroundTag":null,"analyzedSha":"e6b18ccea71451a55fcd861d7b96fddf2587b09a","analyzedAt":"2026-08-14T20:05:26.414Z","schemaVersion":2},"datasetVersion":"2026-08-15T17:31:12.345Z"}