Jinja Template in tokenizer

Base

在transformers中的tokenizer往往会有，以Qwen2.5-7B-Instruct为例，其tokenizer_config.json中有存在chat_template的键，其值如下：

plaintext
"chat_template": "{%- if tools %}\n    {{- '<|im_start|>system\\n' }}\n    {%- if messages[0]['role'] == 'system' %}\n        {{- messages[0]['content'] }}\n    {%- else %}\n        {{- 'You are Qwen, created by Alibaba Cloud. You are a helpful assistant.' }}\n    {%- endif %}\n    {{- \"\\n\\n# Tools\\n\\nYou may call one or more functions to assist with the user query.\\n\\nYou are provided with function signatures within <tools></tools> XML tags:\\n<tools>\" }}\n    {%- for tool in tools %}\n        {{- \"\\n\" }}\n        {{- tool | tojson }}\n    {%- endfor %}\n    {{- \"\\n</tools>\\n\\nFor each function call, return a json object with function name and arguments within <tool_call></tool_call> XML tags:\\n<tool_call>\\n{\\\"name\\\": <function-name>, \\\"arguments\\\": <args-json-object>}\\n</tool_call><|im_end|>\\n\" }}\n{%- else %}\n    {%- if messages[0]['role'] == 'system' %}\n        {{- '<|im_start|>system\\n' + messages[0]['content'] + '<|im_end|>\\n' }}\n    {%- else %}\n        {{- '<|im_start|>system\\nYou are Qwen, created by Alibaba Cloud. You are a helpful assistant.<|im_end|>\\n' }}\n    {%- endif %}\n{%- endif %}\n{%- for message in messages %}\n    {%- if (message.role == \"user\") or (message.role == \"system\" and not loop.first) or (message.role == \"assistant\" and not message.tool_calls) %}\n        {{- '<|im_start|>' + message.role + '\\n' + message.content + '<|im_end|>' + '\\n' }}\n    {%- elif message.role == \"assistant\" %}\n        {{- '<|im_start|>' + message.role }}\n        {%- if message.content %}\n            {{- '\\n' + message.content }}\n        {%- endif %}\n        {%- for tool_call in message.tool_calls %}\n            {%- if tool_call.function is defined %}\n                {%- set tool_call = tool_call.function %}\n            {%- endif %}\n            {{- '\\n<tool_call>\\n{\"name\": \"' }}\n            {{- tool_call.name }}\n            {{- '\", \"arguments\": ' }}\n            {{- tool_call.arguments | tojson }}\n            {{- '}\\n</tool_call>' }}\n        {%- endfor %}\n        {{- '<|im_end|>\\n' }}\n    {%- elif message.role == \"tool\" %}\n        {%- if (loop.index0 == 0) or (messages[loop.index0 - 1].role != \"tool\") %}\n            {{- '<|im_start|>user' }}\n        {%- endif %}\n        {{- '\\n<tool_response>\\n' }}\n        {{- message.content }}\n        {{- '\\n</tool_response>' }}\n        {%- if loop.last or (messages[loop.index0 + 1].role != \"tool\") %}\n            {{- '<|im_end|>\\n' }}\n        {%- endif %}\n    {%- endif %}\n{%- endfor %}\n{%- if add_generation_prompt %}\n    {{- '<|im_start|>assistant\\n' }}\n{%- endif %}\n",

该模版是用于tokenizer.apply_chat_template函数中，通过调用该模版，将多轮对话的messages变成字符串，如下所示。

python
from transformers import AutoTokenizer
tokenizer = AutoTokenizer.from_pretrained("/data1/HF-Models/Qwen/Qwen2.5-7B-Instruct")

chat = [
  {"role": "user", "content": "Hello, how are you?"},
  {"role": "assistant", "content": "I'm doing great. How can I help you today?"},
  {"role": "user", "content": "I'd like to show off how chat templating works!"},
]

tokenizer.apply_chat_template(chat, tokenize=False)

输出如下：

plaintext
<|im_start|>system
You are Qwen, created by Alibaba Cloud. You are a helpful assistant.<|im_end|>
<|im_start|>user
Hello, how are you?<|im_end|>
<|im_start|>assistant
I'm doing great. How can I help you today?<|im_end|>
<|im_start|>user
I'd like to show off how chat templating works!<|im_end|>

如果设置，add_generation_prompt=True，则打印结果如下：

plaintext
<|im_start|>system
You are Qwen, created by Alibaba Cloud. You are a helpful assistant.<|im_end|>
<|im_start|>user
Hello, how are you?<|im_end|>
<|im_start|>assistant
I'm doing great. How can I help you today?<|im_end|>
<|im_start|>user
I'd like to show off how chat templating works!<|im_end|>
<|im_start|>assistant

Advanced: Adding and editing chat templates

很简单，只需编写一个 jinja 模板并设置 tokenizer.chat_template。您可能会发现，从另一个模型的现有模板开始，然后根据您的需要简单地对其进行编辑会更容易！例如，我们可以采用上面的 LLaMA 模板，并将 “[ASST]” 和 “[/ASST]” 添加到助手消息中：

python
template = tokenizer.chat_template
template = template.replace("SYS", "SYSTEM")  # Change the system token
tokenizer.chat_template = template  # Set the new template
tokenizer.push_to_hub("model_name")  # Upload your new template to the Hub!

References

• HF-Chat Templates: https://huggingface.co/docs/transformers/chat_templating#advanced-extra-inputs-to-chat-templates
• Jinja: https://jinja.palletsprojects.com/en/stable/templates/