How to get the visual length of a text string in python

Instead of rendering into an image buffer and counting pixels, you can calculate width directly by using the font metrics. There doesn't seem to be a font API distributed with core python, but there are plenty of third-party ones in various packages. Here's a pretty complete solution for Adobe font metrics, using matplotlib:

>>> from matplotlib import rcParams
>>> import os.path

>>> afm_filename = os.path.join(rcParams['datapath'], 'fonts', 'afm', 'ptmr8a.afm')
>>>
>>> from matplotlib.afm import AFM
>>> afm = AFM(open(afm_filename, "rb"))
>>> afm.string_width_height('What the heck?')
(6220.0, 694)

The metrics are reported in units of 1/1000 of the scale factor (point size) of the font being used. (Thanks @JacobLee for digging up this information.)

Another possibility is the tkFont module of tkinter. This page documents the function tkFont.Font.measure("some string"), but it seems you need a Tk window before you can use it; so I don't know how practical it is:

# Python 3 names -- see Note below
import tkinter 
from tkinter import font as tkFont

tkinter.Frame().destroy()  # Enough to initialize resources
arial36b = tkFont.Font(family='Arial', size=36, weight='bold')
width = arial36b.measure("How wide is this?")
print(width)  # Prints: 404

Note: In python 2 (and in the page I mentioned above), tkinter is known as Tkinter, and tkinter.font is a top-level module, tkFont:

import Tkinter
import tkFont

If you are using Windows, then the following approach could be used.

It uses the current screen as the output context and calculates the dimensions needed to display the given font at the given point size. It returns a tuple holding the text width and text height:

import ctypes

def GetTextDimensions(text, points, font):
    class SIZE(ctypes.Structure):
        _fields_ = [("cx", ctypes.c_long), ("cy", ctypes.c_long)]

    hdc = ctypes.windll.user32.GetDC(0)
    hfont = ctypes.windll.gdi32.CreateFontA(-points, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, font)
    hfont_old = ctypes.windll.gdi32.SelectObject(hdc, hfont)
    size = SIZE(0, 0)
    ctypes.windll.gdi32.GetTextExtentPoint32A(hdc, text, len(text), ctypes.byref(size))
    ctypes.windll.gdi32.SelectObject(hdc, hfont_old)
    ctypes.windll.gdi32.DeleteObject(hfont)
    return (size.cx, size.cy)

for text, font in [
    ('....', 'Arial'), 
    ('WWWW', 'Arial'), 
    ('WWWW', 'Arial Narrow'),
    ('....', 'Courier New'), 
    ('WWWW', 'Courier New'), 
    ("Test", "Unknown font"),
    ('Test', 'Calibri')]:

    print '{:8} {:20} {}'.format(text, font, GetTextDimensions(text, 12, font))

This would display the following output:

....     Arial                (12, 15)
WWWW     Arial                (44, 15)
WWWW     Arial Narrow         (36, 16)
....     Courier New          (28, 15)
WWWW     Courier New          (28, 15)
Test     Unknown font         (24, 15)
Test     Calibri              (23, 14)

Arial being a proportional font shows different dimensions for .... and WWWW but Courier New being fixed width gives the same results. Arial Narrow gives 36 compared to 44 for Arial.

In the case of Unknown font, the Windows font mapper has automatically picked a default font.

Tested on Python 2.x.

---

Note for Python 3.x

As this is calling GetTextExtentPoint32A() in Windows, this expects ANSI text to be passed to it, as such the call could be changed as follows to fix this:

ctypes.windll.gdi32.GetTextExtentPoint32A(hdc, text.encode('cp1252'), len(text), ctypes.byref(size))

Alternatively, switch the code to use the wide versions, replace with these two:

hfont = ctypes.windll.gdi32.CreateFontW(-points, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, font)
ctypes.windll.gdi32.GetTextExtentPoint32W(hdc, text, len(text), ctypes.byref(size))