最基础的字符串模板 PromptTemplate（libs/core/langchain_core/prompts/prompt.py:24）：

# libs/core/langchain_core/prompts/prompt.py:24
class PromptTemplate(StringPromptTemplate):
    """Prompt template for a language model.

    A prompt template consists of a string template. It accepts a set of
    parameters from the user that can be used to generate a prompt for a
    language model.

    The template can be formatted using either f-strings (default), jinja2,
    or mustache syntax.

    Example:
        # Instantiation using from_template (recommended)
        prompt = PromptTemplate.from_template("Say {foo}")
        prompt.format(foo="bar")
    """

填空动作在 format（libs/core/langchain_core/prompts/prompt.py:191），短得可以背下来：

# libs/core/langchain_core/prompts/prompt.py:191
def format(self, **kwargs: Any) -> str:
    """Format the prompt with the inputs."""
    kwargs = self._merge_partial_and_user_variables(**kwargs)   # ① 预填值 + 现填值合并
    return DEFAULT_FORMATTER_MAPPING[self.template_format](self.template, **kwargs)  # ② 按格式渲染

为什么推荐 from_template 而不是直接构造？看源码（libs/core/langchain_core/prompts/prompt.py:257）：

# libs/core/langchain_core/prompts/prompt.py:257
@classmethod
def from_template(cls, template, *, template_format="f-string",
                  partial_variables=None, **kwargs) -> PromptTemplate:
    input_variables = get_template_variables(template, template_format)  # ① ★扫描模板抠出变量名
    partial_variables_ = partial_variables or {}
    if partial_variables_:
        input_variables = [                                              # ② 预填过的不再算"待填空"
            var for var in input_variables if var not in partial_variables_
        ]
    return cls(input_variables=input_variables, template=template,
               template_format=template_format,
               partial_variables=partial_variables_, **kwargs)

聊天模型要消息列表，主角换成 ChatPromptTemplate（libs/core/langchain_core/prompts/chat.py:794）。入口 from_messages（chat.py:1124）接受五花八门的写法：

# libs/core/langchain_core/prompts/chat.py:1124
@classmethod
def from_messages(cls, messages, template_format="f-string") -> ChatPromptTemplate:
    """Create a chat prompt template from a variety of message formats.

    A message can be represented using the following formats:
    1. BaseMessagePromptTemplate
    2. BaseMessage
    3. 2-tuple of (message type, template); e.g., ('human', '{user_input}')
    4. 2-tuple of (message class, template)
    5. A string which is shorthand for ('human', template)
    """
    return cls(messages, template_format=template_format)

真正干活的是 format_messages（libs/core/langchain_core/prompts/chat.py:1174）——逐条"过一遍剧本"：

# libs/core/langchain_core/prompts/chat.py:1174
def format_messages(self, **kwargs: Any) -> list[BaseMessage]:
    kwargs = self._merge_partial_and_user_variables(**kwargs)
    result = []
    for message_template in self.messages:
        if isinstance(message_template, BaseMessage):
            result.extend([message_template])              # ① 写死的台词：原样上
        elif isinstance(message_template,
                        (BaseMessagePromptTemplate, BaseChatPromptTemplate)):
            message = message_template.format_messages(**kwargs)  # ② 带空的台词：填空后上
            result.extend(message)                                #    （可能展开成多条！）
        else:
            raise ValueError(msg)
    return result

MessagesPlaceholder（libs/core/langchain_core/prompts/chat.py:53）就是那行"此处插入前情回顾"的舞台指示。它的 format_messages（chat.py:164）：

# libs/core/langchain_core/prompts/chat.py:164
def format_messages(self, **kwargs: Any) -> list[BaseMessage]:
    value = (
        kwargs.get(self.variable_name, [])      # optional=True：没给就空列表
        if self.optional
        else kwargs[self.variable_name]         # 必填：没给直接 KeyError
    )
    if not isinstance(value, list):
        raise ValueError(msg)                   # 必须是列表，防呆
    value = convert_to_messages(value)          # ★元组简写 → 真消息对象
    if self.n_messages:
        value = value[-self.n_messages :]       # ★只保留最近 n 条（截断历史）
    return value

FewShotPromptTemplate（libs/core/langchain_core/prompts/few_shot.py:121）的 format（few_shot.py:180）把装订过程写得清清楚楚：

# libs/core/langchain_core/prompts/few_shot.py:180
def format(self, **kwargs: Any) -> str:
    kwargs = self._merge_partial_and_user_variables(**kwargs)
    examples = self._get_examples(**kwargs)            # ① 拿例子：固定列表 或 selector 动态挑
    examples = [{k: e[k] for k in self.example_prompt.input_variables} for e in examples]
    example_strings = [
        self.example_prompt.format(**example) for example in examples   # ② 每个例子用小模板渲染
    ]
    pieces = [self.prefix, *example_strings, self.suffix]               # ③ 前言 + 例子们 + 收尾
    template = self.example_separator.join([piece for piece in pieces if piece])
    return DEFAULT_FORMATTER_MAPPING[self.template_format](template, **kwargs)  # ④ 最后整体填空