Python help needed on a loop with multiple variables

Question

Basically I want to iterate 5 times through a list of two variables and populate the output iteratively. I'm probably not describing that well enough, so I'll show what I'm trying to get. I will have a multi-line query in the query2 variable, but I simplified it a bit for the question here.

Here is the code I'm working on:

systems = ["prd1", "prd2", "frz1", "frz2", "dev"]
qgsystems = ["Prd1_v2", "Prd2_v2", "Frz1_v2", "Frz2_v2", "Devl_v2"]
query2 = f"""{{fn blah({x}_1_output.csv)}}select {y};"""
for x, y in zip(systems, qgsystems):
    print(query2)

The output I'm trying to get would be this:

{fn blah(prd1_1_output.csv)}select Prd1_v2;
{fn blah(prd2_1_output.csv)}select Prd2_v2;
{fn blah(frz1_1_output.csv)}select Frz1_v2;
{fn blah(frz2_1_output.csv)}select Frz2_v2;
{fn blah(dev_1_output.csv)}select Devl_v2;

But instead I'm getting this:

{fn blah(dev_1_output.csv)}select Devl_v2;
{fn blah(dev_1_output.csv)}select Devl_v2;
{fn blah(dev_1_output.csv)}select Devl_v2;
{fn blah(dev_1_output.csv)}select Devl_v2;
{fn blah(dev_1_output.csv)}select Devl_v2;

What am I doing wrong?

When I start a fresh session, I get closer to what's wrong. But I'm kinda stuck still.

---------------------------------------------------------------------------
NameError                                 Traceback (most recent call last)
<ipython-input-1-3ea2eb0d6f53> in <module>
      1 systems = ["prd1", "prd2", "frz1", "frz2", "dev"]
      2 qgsystems = ["Prd1_v2", "Prd2_v2", "Frz1_v2", "Frz2_v2", "Devl_v2"]
----> 3 query2 = f"""{{fn blah({x}_1_output.csv)}}select {y};"""
      4 for x, y in zip(systems, qgsystems):
      5     print(query2)

NameError: name 'x' is not defined

You are referring to variable x where the x is not defined. You may use string formatting instead of fstring. — niaei
– niaei, Commented Apr 22, 2022 at 22:40
Strings are immutable, and the variables inside an f-string are evaluated once in order to produce the string contents at the time the f-string is defined, not every time the string is referenced thereafter. If you use str.format() as MSH suggests, the format() function will give you a new string each time it's called. — Samwise
– Samwise, Commented Apr 22, 2022 at 22:49

MatsLindh · Accepted Answer · 2022-04-22 22:41:30Z

4

You can't define the string outside of the loop at expect it to get filled in when printing it inside. Instead you'll have to build the string inside the loop:

systems = ["prd1", "prd2", "frz1", "frz2", "dev"]
qgsystems = ["Prd1_v2", "Prd2_v2", "Frz1_v2", "Frz2_v2", "Devl_v2"]

for x, y in zip(systems, qgsystems):
    print(f"{{fn blah({x}_1_output.csv)}}select {y};")

Output:

{fn blah(prd1_1_output.csv)}select Prd1_v2;
{fn blah(prd2_1_output.csv)}select Prd2_v2;
{fn blah(frz1_1_output.csv)}select Frz1_v2;
{fn blah(frz2_1_output.csv)}select Frz2_v2;
{fn blah(dev_1_output.csv)}select Devl_v2;

answered Apr 22, 2022 at 22:41

MatsLindh

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fsenart · Accepted Answer · 2022-04-22 22:41:27Z

3

systems = ["prd1", "prd2", "frz1", "frz2", "dev"]
qgsystems = ["Prd1_v2", "Prd2_v2", "Frz1_v2", "Frz2_v2", "Devl_v2"]
query2 = "{{fn blah({x}_1_output.csv)}}select {y};"
for x, y in zip(systems, qgsystems):
    print(query2.format(x=x, y=y))

answered Apr 22, 2022 at 22:41

fsenart

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