CODEWITHSUNDEEP
pythonpython-3.xpsycopg2
skywalkerytx
skywalkerytx

Reputation: 244

In python, how to change the element data type of a list

Working with python3.4.3 & psycopg2

I have the following SQL query

SELECT
    somestring1,
    somestring2,
    float1,
    float2,
    float3
FROM
    table

I want to merge float1,float2,float3 into one single float[] then use UPDATE to get it back to database So I write the following:

res = cur.fetchall()
date = list(map(lambda x: x[0],res))
code = list(map(lambda x: x[1], res))
#vector = list(map(lambda x: list(map(float,x[2:])), res)) # this one works
vector = list(map(lambda x:x[2:],res)) # which won't work
res = list(zip(vector,date,code))
cur.executemany("""
    UPDATE mapped SET
        vector = %s
    WHERE
        date = %s AND
        code = %s
""",res) # error occurs here

The error message:

psycopg2.ProgrammingError: column "vector" is of type double precision[] but expression is of type record
LINE 3:             vector = (16.25, 15.56, 16.07, 133.409279, 15.35...
                         ^
HINT:  You will need to rewrite or cast the expression.

From error message my guess is that when vector is created it is created like some sort of list<Any> rather than list<float>. How can I do that simpler than cast every element to float with map?

Upvotes: 1

Views: 288

Answers (1)

Clodoaldo Neto
Clodoaldo Neto

Reputation: 125214

You are passing a list of tuples. A tuple is adapted by Psycopg to a record. You need to pass a list of lists as a list is adapted to an array:

vector = list(map(lambda x:list(x[2:]),res))

Upvotes: 1

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