using map and lambdas in python 3, why this code wont update sql tabel-CodePudding

The purpose of this script is to learn how to use lambda functions with map. I tried to insert a string to all columns of an sqlite TABEL and no error is shown yet the values didn't change.

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))))

CodePudding user response：

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.

CodePudding user response：

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())")