Breaking string into multiple lines according to character width (python)

Question

I am drawing text atop a base image via PIL. One of the requirements is for it to overflow to the next line(s) if the combined width of all characters exceeds the w

Answer 1

If precision matters for you, the best way to get the real text width is to actually render it, since font metrics are not always linear, regarding the kerning or the font size (see here) for example, and therefore not easily predictable. We can approach the optimal breakpoint with ImageFont method get_size that internally uses core font rendering methods (see PIL github)

def break_text(txt, font, max_width):

    # We share the subset to remember the last finest guess over 
    # the text breakpoint and make it faster
    subset = len(txt)
    letter_size = None

    text_size = len(txt)
    while text_size > 0:

        # Let's find the appropriate subset size
        while True:
            width, height = font.getsize(txt[:subset])
            letter_size = width / subset

            # min/max(..., subset +/- 1) are to avoid looping infinitely over a wrong value
            if width < max_width - letter_size and text_size >= subset: # Too short
                subset = max(int(max_width * subset / width), subset + 1)
            elif width > max_width: # Too large
                subset = min(int(max_width * subset / width), subset - 1)
            else: # Subset fits, we exit
                break

        yield txt[:subset]
        txt = txt[subset:]   
        text_size = len(txt)

and use it like so:

from PIL import Image
from PIL import ImageFont
img = Image.new('RGBA', (100, 100), (255,255,255,0))
draw = ImageDraw.Draw(img)
font = ImageFont.truetype("Helvetica", 12)
text = "This is a sample text to break because it is too long for the image"

for i, line in enumerate(break_text(text, font, 100)):
    draw.text((0, 16*i), line, (255,255,255), font=font)

Answer 2

Since you know the width of each character, you should make that into a dictionary, from which you get the widths to calculate the stringwidth:

char_widths = {
    'a': 9,
    'b': 11,
    'c': 13,
    # ...and so on
}

From here you can lookup each letter and use that sum to check your width:

current_width = sum([char_widths[letter] for letter in word])