Python KeyError when printing a string

Question

I'm trying to print a string via stdout in Python which will then be picked up by through a child process in NodeJS. My code is as follows:

sys.stdout.write('{r: {0}, g: {1}, b: {2}, brightness: {3}}'.format(int(result_color.color[0]), int(result_color.color[1]), int(result_color.color[2]), int(current_brightness)))
sys.stdout.flush()

but for some reason I get the following error:

Traceback (most recent call last):
  File "C:\..\script.py", line 811, in <module>
    main(sys.argv[1:])
  File "C:\..\script.py", line 756, in main
    sys.stdout.write('{r: {0}, g: {1}, b: {2}, brightness: {3}}'.format(int(result_color.color[0]), int(result_color.color[1]), int(result_color.color[2]), int(current_brightness)))
KeyError: 'r'

I can surround the keys with a " ' " but then it is not in the correct format to use JSON.parse() through JS. Why is it exhibiting this behavior? It can't think of a reason for it to be trying to process it as a dictionary when I'm passing print a string. (I want to process this as a string through Python and an Object once received in JS. A dict shouldn't be involved on Python's endd)

In the future, check out the json module. In particular, its json.dumps takes a dict and returns a string that can be parsed by JSON.parse on the NodeJS side. This is safer and more readable than string formatting. — GeeTransit
– GeeTransit, Commented Dec 15, 2020 at 1:58

Keijack · Accepted Answer · 2020-12-15 01:50:37Z

3

{ and } is the key characters in a string template, you have to escape it with {{ and }}, Try

sys.stdout.write('{{r: {0}, g: {1}, b: {2}, brightness: {3}}}'.format(int(result_color.color[0]), int(result_color.color[1]), int(result_color.color[2]), int(current_brightness)))

You can find more information here: https://docs.python.org/3/library/string.html#format-string-syntax

edited Dec 15, 2020 at 1:50

answered Dec 15, 2020 at 1:39

Keijack

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Comments

Daniil Loban · Accepted Answer · 2020-12-15 02:32:31Z

0

If you use Python 3.x You can use f-string for fomatting, I think it's more readable

import sys;

class Color:
  def __init__(self, rgb, brightness):
    self.color = rgb
    self.brightness = brightness;

result_color = Color([11,22,33], 100)    

def show_color(color, brightness):
  [r,g,b] = color;
  sys.stdout.write(f'r: {r}, g: {g}, b: {b}, brightness: {brightness}\n')
  sys.stdout.flush()

show_color(result_color.color, result_color.brightness)

answered Dec 15, 2020 at 2:32

Daniil Loban

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GeeTransit · Accepted Answer · 2020-12-15 03:20:47Z

0

Here's an expanded version of my comment. It uses the builtin json module. I've also shortened the code so it looks neater and doesn't repeat as much.

import sys
import json

...

r, g, b = map(int, result_color.color)
data = dict(r=r, g=g, b=b)

sys.stdout.write(json.dumps(data))
sys.stdout.flush()

Also, I suggest that you use the normal print function. If you need to not have a newline at the end and you want to flush it, both are parameters you can specify.

print(json.dumps(data), end="", flush=True)

answered Dec 15, 2020 at 3:20

GeeTransit

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