converting types returned from fetchall()

Question

I'm using psycopg2, and I run the following code: ``` conn = psycopg2.connect(database = mydb_name, host = mydb_server, user = mydb_uname, password = mydb_pwd) cur = conn.cursor() cur.execute("SELECT desiredParams FROM tableOfInterest;") all_data = cur.fetchall() nlargest = heapq.nlargest(5, all_data) ``` This returns a list of tuples with decimals: [(Decimal('254.000'),), (Decimal('154.000'),), (Decimal('244.000'),), (Decimal('134.000'),), (Decimal('254.000'),)] How can I convert this into something more like: [254.000, 154.000, 244.000, 134.000, 254.000] ?

Accepted Answer

Decimals are a fix point number that match nicely with database number types. They are really useful as is, but if I cant convince you to use them.

```
largest = [float(d[0]) for d in nlargest]
```

would give

```
[254.0, 154.0, 244.0, 134.0, 254.0]
```

or even better since cur is a iterible

```
cur.execute("SELECT desiredParams FROM tableOfInterest;")
nlargest = [float(g[0]) for g in heapq.nlargest(5, cur)]
```

In an attempt to make it pretty

```
cur.execute("SELECT param FROM tableOfInterest ORDER BY param DESC FIRST 5;")
nlargest = [float(r.param) for r in cur]
```

or skip the list building and just do it

```
cur.execute("SELECT param FROM tableOfInterest ORDER BY param DESC FIRST 5;")
for param, in cur:
    stuff_to_do_with_first_5(float(param))
```

the way to bind that looks good but is really bad (due to pyscopg's mishandling of bind variables). The problem with this is that pysco just % the values into the string and thus hides the ability to inject sql into your string.

```
cur.execute("SELECT param FROM tableOfInterest ORDER BY param DESC FIRST %s;", (num_results,))
```

the way that looks bad but is safer then the previous

```
cur.execute("SELECT param FROM tableOfInterest ORDER BY param DESC FIRST %d;" % num_results)
```

converting types returned from fetchall()

Solution

Problem