learning to use map and lambda in python 3

Question

The purpose of this script is to learn how to use `lambda` functions in conjunction with `map`.

I tried to insert a string to all columns of an sqlite table and no error is shown yet the values didn't change.


class DB(SomeDbBase):

    def get_columns(self):
        res = self.get_cursor().execute("SELECT * FROM EVENTS")
        names = list(map(lambda x: x[0], res.description))
        return names`


    def update_to_last(self,column:str,data:str):
        c = self.get_cursor()
        print(column,data)
        c.execute(
                 f"UPDATE EVENTS SET 
                 '{column}'='{data}' WHERE ID ='(SELECT last_insert_rowid())'")
        self.conn.commit()

    if __name__ == "__main__":
        d=DB()
        columns=d.get_columns()

        # this pile of map and lambda's ment to first get all of the columns names
        # then add to every string a pair of some fictionary "data" to a list
        # then the list is sent to update
        map(
           lambda x:d.update_to_last(x[0],x[1]),
           (list(map(lambda column:[column,"data"],columns)))
           )

don't use map for side effects. That is confusing and unidiomatic. Just use a regular for-loop. — juanpa.arrivillaga
– juanpa.arrivillaga, Commented Dec 20, 2021 at 22:58

chepner · Accepted Answer · 2021-12-20 22:37:45Z

3

map does not create a list of results. It produces items on demand as you iterate over the map instance. You aren't iterating over the map instance, so update_to_last never gets called on anything.

Don't use map for side effects. Use a regular for loop.

# No need to turn the map instance into a list first; you
# can iterate over the map directly.
for x in map(lambda column: [column, "data"], columns):
    d.update_to_last(x[0], x[1])

Of course, since "data" is the fixed value of x[1], you really don't need map at all.

for column in columns;
    d.update_to_last(column, "data")

On the other hand, if d.update_to_last produced an interesting return value that you wanted to store in a list, you might do somethign like

from itertools import repeat
x = list(map(d.list_to_update, columns, repeat("data")))

although a list comprehension like

x = [d.list_to_update(c, "data") for c in columns]

would be preferable.

edited Dec 20, 2021 at 22:37

answered Dec 20, 2021 at 22:25

chepner

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2 Comments

user16776498 Over a year ago

thanks, learnt a lot, but theoretically if i cast map to list it forces the map to yield until iteration exception is reached, am i right? if so then it would call d.update_to_last(column, "data") for all the iterations?

chepner Over a year ago

Yes, list just does all the iteration at once in order to build a list. The point of using something like map is if you don't need all the items in memory at once, but instead just need the first one, which you can then discard before getting the second one, etc. You'll use a constant amount of memory rather than a linear amount.

Muhammed Jaseem · Accepted Answer · 2021-12-20 22:19:44Z

0

Did you mean something like this? (change the column name id accordingly)

c.execute(f"UPDATE EVENTS SET '{column}'='{data}' WHERE id = (SELECT last_insert_rowid())")

answered Dec 20, 2021 at 22:19

Muhammed Jaseem

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2 Comments

kosciej16 Over a year ago

The opp wanted to learn map and lambda, how your answer helps with that?

Muhammed Jaseem Over a year ago

Well, I think the issue is in the SQL statement instead of map and lambda

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The purpose of this script is to learn how to use `lambda` functions in conjunction with `map`.