Passing a member function as a function arg?

Question

I've written a very simple select function for SQLite, but I'm confused at how to pass a member function... e.g.: .fetchone(), .fetchmany().

def select(cursor, select="*", table="reuters", fetch=".fetchone()", tologfile=False, logfile=""):
    if tologfile:
        logfile = open(logfile, 'w')
        logfile.write(str(cursor.execute("select * from ?;".replace('?',table).replace("select * ", "select "+select)).fetchone()))
        logfile.close()
    else: return str(cursor.execute("select * from ?;".replace('?',table).replace("select * ", "select "+select)).fetchone())

How do I pass this member function as an arg?

ThiefMaster · Answer 1 · 2012-06-07T08:37:08.017

3

You can simply pass self.fetchone to pass that function.

If you want it as a default value simply use None in the function definition and add

if whatever is None:
    whatever = self.fetchone

in the function itself.

If you want to call the method on another object but self keep passing it as a string and use this code (based on your else code since that one's shorter):

result = self.execute("select * from ?;".replace('?',table).replace("select * ", ("select "+attr)))
return str(getattr(result, whatever)())

edited Jun 07 '12 at 08:37

answered Jun 07 '12 at 08:23

ThiefMaster

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Ah, you want to call that method on the object inside the query.. misunderstood your question – ThiefMaster Jun 07 '12 at 08:35
No worries, looking forward to reading your modified answer :) – user1438003 Jun 07 '12 at 08:36
1

Why not pass the unbound method (Class.method) around and call that, passing the desired instance as the first argument? No need to pass the name of the method around as a string. – Pieter Witvoet Jun 07 '12 at 10:32

score 2 · Answer 2 · answered Jun 07 '12 at 08:14

2

You could use getattr :

>>> class A:
...     def b(self):
...             print 'c'
... 
>>> a = A()
>>> getattr(a,'b')
<bound method A.b of <__main__.A instance at 0x7f2a24a85170>>
>>> getattr(a,'b')()
c

answered Jun 07 '12 at 08:14

Zashas

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cjh · Answer 3 · 2012-06-07T08:22:34.920

0

A lambda could achieve this

class A:
  def test(self):
    print "hello world"

a = A()
func = (lambda: a.test())
func()

prints "hello world"

This technique can also be extended to deal with passing and transforming arguments

class B:
  def test(self, x):
    print x

b = B()
func = (lambda a, b : b.test(b))
func("garbage", "foo")

prints "foo"

edited Jun 07 '12 at 08:22

answered Jun 07 '12 at 08:16

cjh

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1
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Or, you could just use `func = a.test`. The lambda is unnecessary. – Ben Jun 08 '12 at 02:22

user1438003 · Accepted Answer · 2012-06-07T09:12:28.737

Okay, got it to work:

import sqlite3

def select(self, attr="*", table="reuters", fetch=None, num=None, tologfile=False, logfile=""):
    if fetch is None:
        fetch=self.fetchone
    output=self.execute("select * from ?;".replace('?',table).replace("select * ", ("select "+attr+' ')))

    output=fetch(num) if num else fetch()

    if tologfile:
        logfile = open(logfile, 'w')
        logfile.write(str(output))
        logfile.close()
    else: return output

if __name__ == '__main__':    
    connection = sqlite3.connect('winkwinknudgenudgesaynomore.db')
    cursor = connection.cursor()
    cursor.execute("drop table reuters;")
    cursor.execute("create table reuters (foo text, bar text);")
    connection.commit()
    print select(cursor)
    print select(cursor, 'bar')
    print select(cursor, 'bar', fetch=cursor.fetchmany, num=5)
    cursor.close()

Passing a member function as a function arg?

4 Answers4