local_context() blocks appear to "leak" into one another

[smitelli]

When using local_context() in the main document and header/footer methods, the parameters set in the document leak into the header/footer in unexpected ways:

from fpdf import FPDF


class SubclassFPDF(FPDF):
    def header(self):
        with self.local_context(text_color='#00ff00', font_size=12):  # LABEL A
            self.cell(text=f'Header {self.page_no()}')
        self.ln()

    def footer(self):
        self.set_y(-15)
        with self.local_context(text_color='#0000ff', font_size=12):  # LABEL B
            self.cell(text=f'Footer {self.page_no()}')


pdf = SubclassFPDF()
pdf.set_font(family='helvetica', size=12)
pdf.add_page()

with pdf.local_context(font_size=36):  # LABEL C
    pdf.multi_cell(w=0, text='\n'.join(f'Line {i + 1}' for i in range(50)))

pdf.output('testcase.pdf')

The expected output is that all header/footer text is in font size 12. The actual output is that the 36pt font defined at label C leaks into the footer() function and overrides the 12pt font defined at label B -- but this only happens on the last page of the document. There is no obvious way to make the font size at B take effect consistently without removing the set_context() at C.

Furthermore, based on my understanding of the set_context() philosophy (briefly... any values set explicitly will take effect, unspecified values continue to use whatever the outer environment had, and when the context exits everything is exactly the way is was before the context was entered) I would think I don't need the explicit font_size=12 at labels A or B at all to use the main document font size. But omitting them causes all headers/footers after the top of page 1 to get the 36pt font from C.

This is fpdf2 version 2.7.9 on both Python 3.9.16 and 3.12.3.

[smitelli]

Even this is not enough to get the footer on page 3 to use 12pt font:

    def footer(self):
        self.set_y(-15)
        self.set_font_size(12)
        with self.local_context(text_color='#0000ff', font_size=12):
            self.set_font_size(12)
            self.cell(text=f'Footer {self.page_no()}')

[Lucas-C]

Hi @smitelli

Thank you for the detailed bug report.
I'm having a look at this now

[Lucas-C]

@allcontributors please add @smitelli for bug

[allcontributors]

@Lucas-C

I've put up a pull request to add @smitelli! 🎉

[Lucas-C]

This is indeed a bug.
I'm trying to figure the best way to fix this...

In the meantime, for this use case, I suggest that use you use the use_font_face() method instead, which is more reliable for now:

from fpdf import FPDF, FontFace


class PDF(FPDF):
    def header(self):
        with self.use_font_face(FontFace(color="#00ff00", size_pt=12)):  # LABEL A
            self.cell(text=f"Header {self.page_no()}")
        self.ln()

    def footer(self):
        self.set_y(-15)
        with self.use_font_face(FontFace(color="#0000ff", size_pt=12)):  # LABEL B
            self.cell(text=f"Footer {self.page_no()}")

pdf = PDF()
pdf.set_font(family="helvetica", size=12)
pdf.add_page()
with pdf.use_font_face(FontFace(size_pt=36)):  # LABEL C
    pdf.multi_cell(w=0, text="\n".join(f"Line {i + 1}" for i in range(21)))

[Lucas-C]

I opened PR #1207 to fix this bug.

@smitelli If you want, you can test this fix by installing fpdf2 from the master branch:

pip install git+https://github.com/py-pdf/fpdf2.git@master

With this fix, this code should now work as expected, with only a single call to .local_context() needed:

from fpdf import FPDF

class PDF(FPDF):
    def header(self):
        self.cell(text=f"Header {self.page_no()}")
        self.ln()

    def footer(self):
        self.set_y(-15)
        self.cell(text=f"Footer {self.page_no()}")

pdf = PDF()
pdf.set_font(family="helvetica", size=12)
pdf.add_page()
with pdf.local_context(font_size_pt=36):  # LABEL C
    pdf.multi_cell(w=0, text="\n".join(f"Line {i + 1}" for i in range(21)))
pdf.output('testcase.pdf')