Use a more robust method for calculating the width of strings

Oct 13, 2025 by wbrum.org 96 views

Introduction

When working with the FPDF library in Python, developers often encounter issues with the get_string_width function and its impact on the multi_cell function. In this article, we will delve into the problem of the width calculated by get_string_width being slightly too small for multi_cell, and explore possible solutions to this issue.

Understanding the Problem

The problem arises when using the get_string_width function to calculate the width of a string, and then passing this value to the multi_cell function. Despite setting the width to an exact value, the multi_cell function may still insert a line break, resulting in the string being split into multiple lines. This issue is particularly finicky and can be difficult to reproduce, as small changes in the text, font, or page unit can cause the problem to disappear.

Error Details

The error details reveal that the get_string_width function returns a value that is slightly too small, leading to the multi_cell function inserting a line break. This is not due to the c_margin value, which has been set to 0 and included in the calculation. The issue is further complicated by the fact that the MultiLineBreak function calculates the width as a floating-point number, which can lead to rounding errors.

Minimal Code

To reproduce the issue, we can use the following minimal code snippet:

from fpdf import FPDF
from fpdf.enums import MethodReturnValue

doc = FPDF(unit="pt")
doc.add_font("Inter", fname="./Inter.ttc")
doc.set_font("Inter")
doc.add_page()
# the bug occurs even if not setting c_margin to 0, since it is included in the length below
doc.c_margin = 0
text = "2025-03-11 17:42 UTC"
length = doc.get_string_width(text) + doc.c_margin * 2
# length = 127.512
print(f"length = {length}")
lines = doc.multi_cell(
    text=text,
    dry_run=True,
    output=MethodReturnValue.LINES,
    w=length,
)

# this fails
assert len(lines) == 1, lines

Environment

The issue has been reproduced on the following environment:

Operating System: Windows 11
Python version: 3.9.21
fpdf2 version used: 2.8.2

Is this Fixable?

The issue is likely due to a classic floating-point error, where the MultiLineBreak function calculates the width as a floating-point number, which can lead to rounding errors. This means that the fix may not be straightforward, and may require a deeper understanding of the FPDF library and its underlying algorithms.

Workaround

One possible workaround is to round up the values returned by the get_string_width function. This can be achieved using the math.ceil function, which returns the smallest integer not less than the given number.

import math

length = math.ceil(doc.get_string_width(text)) + doc.c_margin * 2

However, this workaround may not be ideal, as it may introduce additional errors or inconsistencies in the layout of the document.

Conclusion

In conclusion, the issue of the width calculated by get_string_width being slightly too small for multi_cell is a complex problem that requires a deep understanding of the FPDF library and its underlying algorithms. While a workaround may be possible, it is essential to investigate the root cause of the issue and explore alternative solutions that do not rely on rounding up the values returned by get_string_width.

Recommendations

Based on the analysis of the issue, the following recommendations can be made:

Use a more robust method for calculating the width of strings: Instead of relying on the get_string_width function, consider using a more robust method, such as using a library that provides accurate string width calculations.
Avoid using floating-point numbers for layout calculations: When performing layout calculations, it is generally safer to use integer values instead of floating-point numbers, which can lead to rounding errors.
Investigate alternative solutions: Explore alternative solutions that do not rely on rounding up the values returned by get_string_width, such as using a different font or adjusting the page unit.

Q: What is the issue with the width calculated by get_string_width?

A: The issue is that the width calculated by get_string_width is slightly too small, leading to the multi_cell function inserting a line break and splitting the string into multiple lines.

Q: Why is this issue so finicky?

A: This issue is finicky because small changes in the text, font, or page unit can cause the problem to disappear. This is due to the fact that the MultiLineBreak function calculates the width as a floating-point number, which can lead to rounding errors.

Q: Is this issue specific to the FPDF library?

A: No, this issue is not specific to the FPDF library. It is a general problem that can occur when working with floating-point numbers and layout calculations.

Q: What is the recommended solution to this issue?

A: The recommended solution is to use a more robust method for calculating the width of strings, such as using a library that provides accurate string width calculations. Alternatively, you can use a different font or adjust the page unit to avoid the issue.

Q: Can I use a workaround to fix this issue?

A: Yes, one possible workaround is to round up the values returned by the get_string_width function using the math.ceil function. However, this workaround may not be ideal, as it may introduce additional errors or inconsistencies in the layout of the document.

Q: How can I avoid this issue in the future?

A: To avoid this issue in the future, you can follow these best practices:

Use a more robust method for calculating the width of strings.
Avoid using floating-point numbers for layout calculations.
Investigate alternative solutions that do not rely on rounding up the values returned by get_string_width.

Q: What are some alternative solutions to this issue?

A: Some alternative solutions to this issue include:

Using a different font that is more accurate for layout calculations.
Adjusting the page unit to avoid the issue.
Using a library that provides accurate string width calculations.
Implementing a custom solution using a different algorithm or approach.

Q: Can I report this issue to the FPDF library developers?

A: Yes, you can report this issue to the FPDF library developers. They may be able to provide a fix or a workaround for this issue.

Q: How can I get help with this issue?

A: You can get help with this issue by:

Checking the FPDF library documentation and forums for similar issues.
Searching online for solutions and workarounds.
Reaching out to the FPDF library developers or community for assistance.
Posting a question on a programming forum or Stack Overflow.