Type checking strictness

[pandvan]

I'm trying to render the following template with liquidpy.

{{ 'now' | date: '%s' | times: 1000 | minus: 1800000 }}

It should be valid Liquid, it's properly rendered using various online tester tools.
Using liquidpy leads to a TypeError

TypeError: unsupported operand type(s) for *: 'DateTime' and 'int'

as date are managed via DateTime class that only provide plus and minus operations at the moment.
I don't know how Liquid handles data types. Is there any way I can force/cast to a specific data type (int in this case), "bypassing" Python type checking and so make it works?

p.s. I hit this liquidpy issue as I'm a user of Shuffle automation platform.

[pwwang]

Related: #38

There is a difference in type handling between ruby liquid and liquidpy:

You can do this in ruby liquid:

{{ "1" | plus: 1 }}  # 2

However, this is not valid in liquidpy. Because the template is eventually compiled into python code and the type handling is delegated to python, but "1" + 1 is not a valid python operation.

To solve this, a Datetime class was introduced, which renders the date into string by given format, while handling plus and minus by regarding the object as a python datetime object (#38). However (@frikky), there is a misunderstanding about the issue. After the arithmetic operation, the object should be turned into numbers, instead of a Datetime object.

This should still be a valid template to render:

{{ "now" | date: "%s" | plus: 86400 | date: "%Y-%m-%d" }}

But now it is not.

So, the plus, minus and other arithmetic operations managed by Datetime will return numbers in the next version to make the above template working.

However, {{ "now" | date: "%s" }} should hold a string instead of a number, literally, just like the very first simple template I showed. Putting it in an arithmetic operation directly confuses people since that simple one doesn't work.

A universal way to do it is to convert the string into a number explicitly:

{{ "1" | int | plus: 1}}  # 2

{{ 'now' | date: '%s' | int | times: 1000 | minus: 1800000 }}
# 1634141733000

Now, because the typecasting is handled externally (by the filter int), other than internally, by the Datetime class, so that Datetime class is not necessary to exist anymore.

It will still be available to do an arithmetic operation in the next minor release (0.7.2) with the output from date:

{{ "now" | date: "%s" | plus: 86400 }}  # 1634230267

However, we should direct people to handle the typecasting themselves to avoid confusion.

[pandvan]

You can do this in ruby liquid:

{{ "1" | plus: 1 }}  # 2

However, this is not valid in liquidpy. Because the template is eventually compiled into python code and the type handling is delegated to python, but "1" + 1 is not a valid python operation.

Just hit a similar issue. Capture tag results in a string and so it's not possible to do some arithmetic operation after.
A little example:

{% capture lst_size %}4{% endcapture %}
{{ 2 | at_most: lst_size }}

A universal way to do it is to convert the string into a number explicitly:

{{ "1" | int | plus: 1}}  # 2

However, we should direct people to handle the typecasting themselves to avoid confusion.

I'm totally agree that explicit typecasting specified by the user could be the best solution.
Do you think is viable to add those typecasting filters in liquidpy?

[pwwang]

Yes, at the next release (0.7.2), int, float, str and bool are added as filters and globals. So you should be able to do this:

{% capture lst_size | int %}4{% endcapture %}
{{ 2 | at_most: lst_size }}

or use int as a global:

{% capture lst_size %}4{% endcapture %}
{{ 2 | at_most: int(lst_size) }}     

See also the docs just added: #41 (comment)

[pwwang]

0.7.2 is released.