fpdf.ln() not working in version 2.5.7 if last fpdf.cell() was called with empty txt

[rgarus]

In the following example line feed should use as height the height value of the last printed cell, which is the height of the current font size. This works in fpdf2 version 2.5.6, but no longer in version 2.5.7. In version 2.5.7 the used height value is 0.

Minimal code

from fpdf import FPDF

pdf = FPDF()
pdf.add_page()
pdf.set_font("Times", size=12)

for i in range(1, 10):
    pdf.cell(w=100, h=None, txt=f"Printing line number {i}")
    pdf.cell(w=100, h=None, txt="")
    pdf.ln()

pdf.output("test.pdf") 

Environment

• OS Linux
• Python version 3.10
• fpdf2 version 2.5.7

test_2.5.6.pdf
test_2.5.7.pdf

[gmischler]

Thanks for reporting this!

Looks like it's my fault and happened in #519.
A tactical fix should be easy and quick, I will look at it more closely tonight.

On a strategic level though, I'm not sure about the wisdom of maintaining self.lasth for this purpose.
@Lucas-C, wouldn't it make more sense to simply use the current font height?
It doesn't look like self.lasth is documented anywhere outside of one short code comment, so it's unlikely that many people will miss it.

[Lucas-C]

On a strategic level though, I'm not sure about the wisdom of maintaining self.lasth for this purpose.

lasth is now private: https://github.com/PyFPDF/fpdf2/blob/master/fpdf/fpdf.py#L266
It is set by _render_styled_text_line() called from text(), cell(), multi_cell() & write().
My understanding is that its value does not always come from self.font_height. It can be specified for example from the h parameter of cell().

Hence, while I would be OK to get rid of self._lasth, I do not see how to do so without impacting the library behaviour.

If you can submit a PR to fix the issue @gmischler, it would be very welcome 😊

[gmischler]

Hence, while I would be OK to get rid of self._lasth, I do not see how to do so without impacting the library behaviour.

Since it is not currently possible to change the font size within one of the relevant methods, I don't think that self._lasth can actually differ from the current font size (except through this bug).
Either way, the fix should be quite straightforward.

[Lucas-C]

Since it is not currently possible to change the font size within one of the relevant methods, I don't think that self._lasth can actually differ from the current font size (except through this bug).

@gmischler: consider the following code

from fpdf import FPDF

pdf = FPDF()
pdf.add_page()
pdf.set_font("helvetica", size=18)
pdf.cell(txt="Hello world")
print(pdf._lasth)  # prints: 6.35
pdf.cell(txt="Hello world", h=50)
print(pdf._lasth)  # prints: 50

As you can see self._lasth can change even if .font_size does not.

[gmischler]

As you can see self._lasth can change even if .font_size does not.

Ah I see.
To be honest, this feels like a rather weird behaviour to me, but it also appears to be rather old, so we'll have to live with it for now...