Python syntax - using parentheses with indentation

Question

I'm taking a course on Polars and found the syntax I don't fully understand. Here's sample from the course:

csvFile = "..\\data\\example.csv"
(
    pl.read_csv(csvFile)
    .filter(pl.col("Col").is_null())
)

this is working. When I try to modify the line to

csvFile = "..\\data\\example.csv"
ex=(
    pl.read_csv(csvFile)
    .filter(pl.col("Col").is_null())
    
)

this is also working. But when I to make further changes to:

csvFile = "..\\data\\example.csv"
(
    ex=pl.read_csv(csvFile)
    .filter(pl.col("Col").is_null())
    
)

I get syntax error ex=pl.read_csv(csvFile) ^ SyntaxError: invalid syntax. Maybe you meant '==' or ':=' instead of '='?

What is the meaning of these parentheses? I really like this kind of syntax but can't find reference on how to use it properly. I'm using python 3.11 in VSCode with Jupyter extension.

Artur

Needed for having the statement over multiple lines without escaping the newlines, see https://stackoverflow.com/questions/53162/how-can-i-do-a-line-break-line-continuation-in-python — ivvija, Mar 22 '23 at 09:54
You can put an *expression* in parentheses, but not a *statement*. So for example `(2+2)` is valid, and so is `x=(2+2)` but `(x=2+2)` is not valid. That's what's different about the last example in your post, compared to the first. — slothrop, Mar 22 '23 at 10:10
If you want to break your second example across lines without using a backslash, you could do `ex=(pl.read_csv(csvFile).filter(pl.col("Col").is_null()))` (note the extra parentheses that surround the part of the statement after the `=` sign). Within that, you can put a line break before any of the `.` method calls that you prefer. — slothrop, Mar 22 '23 at 10:12
Are you familiar with the difference between an expression and a statement? `a = b` is a statement, just like `def f(): pass`, and can’t be parenthesised. Expressions like `pl.read_csv(csvFile) .filter(pl.col("Col").is_null()) ` can be found in all situations a statement can, in addition to being able to be parenthesised and can occur on the right-hand-side of an assignment operation. — dROOOze, Mar 22 '23 at 10:13

score 1 · Accepted Answer · answered Mar 22 '23 at 10:19

That is just a way to put on several lines what should go on a single line. It's just formatting stuff.

Both

(
    expression on 
    several lines
)

and

variable = (
    expression on
    several lines
)

works. In the first case your expression got evaluated but its output is not saved to any variable, in the second it does.

As a general rule, everything that is inside brackets (both round and squared ones) can be safely written on multiple lines for readability purposes, without needing to escape newline with a \ at the end of the lines. PEP8 contains all rules related to a good formatting of python code.

score -1 · Answer 2 · answered Mar 22 '23 at 10:04

-1

A container enclosed by parenthesis is named a tuple. Tuples contain non-mutable order of items inside.

Tuple is similar to a List, where a Tuple is enclosed by () and a List is enclosed by [].

For example:

tuple_1 = (item1, item2, item3)
list_1 = [item1, item2, item3]

answered Mar 22 '23 at 10:04

Mijanur Rahman

1

4

This is completely wrong. There is no trailing comma, so the expression simply evaluates to whatever `pl.read_csv(...).filter(...)` returns, not a tuple. – InSync Mar 22 '23 at 10:07

Python syntax - using parentheses with indentation

2 Answers2