Python .one() 和 .first() 有什么区别
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What is the difference between .one() and .first()
提问by Reena Shirale
What is the difference between oneand firstmethods in SQLAlchemy
SQLAlchemy 中的one和first方法有什么区别
采纳答案by Martijn Pieters
Query.one()requires that there is only oneresult in the result set; it is an error if the database returns 0 or 2 or more results and an exception will be raised.
Query.one()要求结果集中只有一个结果；如果数据库返回 0 或 2 个或更多结果，则为错误并引发异常。
Query.first()returns the first of a potentially larger result set (adding LIMIT 1to the query), or Noneif there were no results. No exception will be raised.
Query.first()返回可能更大的结果集（添加LIMIT 1到查询）中的第一个，或者None如果没有结果。不会引发任何异常。
From the documentation for Query.one():
从文档中Query.one()：
Return exactly one result or raise an exception.
只返回一个结果或引发异常。
and from Query.first():
Return the first result of this Query or Noneif the result doesn't contain any row.
如果结果不包含任何行，则返回此 Query 的第一个结果或 None。
(emphasis mine).
（强调我的）。
In terms of a Python list, one()would be:
就 Python 列表而言，one()将是：
def one(lst):
if not lst:
raise NoResultFound
if len(lst) > 1:
raise MultipleResultsFound
return lst[0]
while first()would be:
而first()将是：
def first(lst):
return lst[0] if lst else None
There is also a Query.one_or_none()method, which raises an exception only if there are multiple results for the query. Otherwise it'll return the single result, or Noneif there were no results.
还有一种Query.one_or_none()方法，仅当查询有多个结果时才会引发异常。否则它将返回单个结果，或者None如果没有结果。
In list terms, that'd be the equivalent of:
在列表中，这相当于：
def one_or_none(lst):
if not lst:
return None
if len(lst) > 1:
raise MultipleResultsFound
return lst[0]
