python - Python 中的参数计数语法错误

Question

我在尝试创建联合表生成器时遇到语法错误。

这是被冒犯的解释器:

Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "federatedTableBuilder.py", line 7, in <module>
     local_public_files.generate()
  File "localViewDefinition.py", line 22, in generate
    self.generate_for_host(conn)
  File "localViewDefinition.py", line 17, in generate_for_host
    self.conn.doQuery(rsaconn,self.query)
TypeError: doQuery() takes exactly 2 arguments (3 given)

和有问题的代码:

import mysql as sql
from connector import Connector
import io
import traceback

class LocalViewDefinition:

    ...insert variables...

    def doQuery(connection, query):
        try:
            cursor = MySQLdb.cursors.Cursor(connection)
            cursor.execute(query)
        except: #figure out how to handle generic and sql errors separately
            traceback.print_exc()

你知道原因吗？

Answer 1

对于类方法，Python 采用额外的第一个参数来引用类的实例。约定是使用单词 self:

def doQuery(self, connection, query):
      try:
          cursor = MySQLdb.cursors.Cursor(connection)
          cursor.execute(query)
      except: #figure out how to handle generic and sql errors separately
          traceback.print_exc()

这种引用类实例的要求是因为 Python 中“显式优于隐式”(参见 import this)。