Pass values from python list to Cassandra query

Question

Beginner here. I have the following circumstances.

A text file with each line containing a name.
A cassandra 3.5 database
A python script

The intention is to have the script read from the file one line (one name) at a time, and query Cassandra with that name.

FYI, everything works fine except for when I try to pass the value of the list to the query.

I current have something like:

#... driver import, datetime imports done above
#...

with open(fname) as f:
content = f.readlines()

# Loop for each line from the number of lines in the name list file
# num_of_lines is already set
for x in range(num_of_lines):
    tagname = str(content[x])

    rows = session.execute("""SELECT * FROM tablename where name = %s and date = %s order by time desc limit 1""", (tagname, startDay))
    for row in rows:
        print row.name + ", " + str(row.date)

Everything works fine if I remove the tagname list component and edit the query itself with a name value.

What am I doing wrong here?

What excpetion are you getting? NameError: name 'tagname' is not defined ? — Chen A.
– Chen A., Commented Sep 13, 2017 at 15:36
Also what is the expected value of tagname = str(content[x])? It might be None, and the root cause. — Chen A.
– Chen A., Commented Sep 13, 2017 at 15:41
@Vinny Unfortunately, I'm not getting any error. The print row.name.... simply isn't printing. No errors at all. And now, tagname = str(content[x]) is not none. print tagname works perfectly. — bipster
– bipster, Commented Sep 13, 2017 at 16:03

Tejas · Accepted Answer · 2017-09-13 16:48:05Z

1

Simply building on the answer from @Vinny above, format simply replaces literal value. You need to put quotes around it.

for x in content:
    rows = session.execute("SELECT * FROM tablename where name ='{}' and date ='{}' order by time desc limit 1".format(x, startDay))
    for row in rows:
        print row.name + ", " + str(row.date)

answered Sep 13, 2017 at 16:48

Tejas

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Comments

Chen A. · Accepted Answer · 2017-09-13 15:48:09Z

0

You can simply iterate over content:

for x in content:

    rows = session.execute("SELECT * FROM tablename where name = {} and date = {} order by time desc limit 1".format(x, startDay))
    for row in rows:
        print row.name + ", " + str(row.date)
    ....

Also, you don't need to have 3 quotes for the string. Single quotes is good enough (3 quotes is used for documentation / multiple line comments in python)

Note that this might end in a different error; but you will be iterating on the lines instead of iterating over an index and reading lines.

edited Sep 13, 2017 at 15:48

answered Sep 13, 2017 at 15:39

Chen A.

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4 Comments

bipster Over a year ago

Thanks. Three quotes, multiple lines. Got it. I modified the code. It helped me avoid having to read the number of lines. In the query, I changed %s to {} as per your snippet. I get the following error now: TypeError: not all arguments converted during string formatting

Chen A. Over a year ago

@bshakya it might be you have a None object. Use debug prints to check the type of tag name print type(x) before your original print

bipster Over a year ago

An observation. print x apparently includes a \n at the end of it. So when i print it, i can see a new line being printed as well. I think that might be the problem? How do I remove that and transform the value into a pure string?

bipster Over a year ago

And the output of print type(x) is: <type 'str'>

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