Python CSV reader doesnt have any data

Question

The following code work fine.

with open(filename, 'rb') as f:
    reader = csv.reader(f)
    for row in reader:
        cur.execute(insertStatement, row)

when I insert these two lines, something goes wrong.

with open(filename, 'rb') as f:
    reader = csv.reader(f)
    totalrows = len(list(reader))
    print totalrows # Print out the correct output
    for row in reader:
        cur.execute(insertStatement, row)

My guess is that when i assign totalrows = len(list(reader)) the cursor moves to the end of the file there for nothing happens in the for loop.

if this is true how can i move the cursor back to the start without closing the file and reopening it? if not please help.

mhawke · Accepted Answer · 2014-06-20 04:41:52Z

4

That's right, the file has been consumed and subsequent reads on reader will not return data.

You can get around this by calling f.seek(0) on the underlying file, i.e.

with open(filename, 'rb') as f:
    reader = csv.reader(f)
    totalrows = len(list(reader))
    print totalrows # Print out the correct output
    f.seek(0)
    for row in reader:
        cur.execute(insertStatement, row)

answered Jun 20, 2014 at 4:41

mhawke

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1 Comment

mhawke Over a year ago

@M.ofCA : Blair makes an important point about not reading the file twice in this answer

Blair · Accepted Answer · 2014-06-20 04:54:03Z

1

Rather than reading it once, creating and throwing away a list, and reading it again, why don't you keep the list and iterate over that?

with open(filename, 'rb') as f:
    reader = csv.reader(f)
    allrows = list(reader)
    totalrows = len(allrows)
    print totalrows # Print out the correct output
    for row in allrows:
        cur.execute(insertStatement, row)

Also, many database libraries allow you to insert multiple records at once which is often quicker than inserting them one at a time. Check the documentation for whatever database library you are using.

answered Jun 20, 2014 at 4:54

Blair

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Comments

GleasonK · Accepted Answer · 2014-06-20 04:46:20Z

0

If that is whats happening, try using f.seek(0) that puts the cursor back to the beginning.

with open(filename, 'rb') as f:
reader = csv.reader(f)
totalrows = len(list(reader))
print totalrows # Print out the correct output
f.seek(0)  # Moves pointer back to beginning of file
for row in reader:
    cur.execute(insertStatement, row)

edited Jun 20, 2014 at 4:46

answered Jun 20, 2014 at 4:41

GleasonK

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