Why is this str.format causing a Key Error in Python

Question

I have the following code in a python 2.7.10 script

params = {'F': '250', 'I': '-22.5', 'J': '-22.5', 'Y': '12.817175976', 'X': '7.4', 'Z': '-50'}
G3 = 'G3 F {F} I {I} J {J} X {X} Y {Y} Z {Z}  \n'
print(params)
print(G3)
print(G3.format(params))

When I try to run it it gives the following output:

./g-codeGenerator.py
{'F': '250', 'I': '-22.5', 'J': '-22.5', 'Y': '12.817175976', 'X': '7.4', 'Z': '-50'}
G3 F {F} I {I} J {J} X {X} Y {Y} Z {Z} 

Traceback (most recent call last):
  **Traceback truncated**
  File "./g-codeGenerator.py", line 342, in siliconOutputSequence
    print(G3.format(params))
KeyError: 'F'

Why is this causing the key error, as far as I can see all of the required elements are present in the parameters?

zondo · Accepted Answer · 2016-03-14 14:51:40Z

5

It's because .format() is not expecting a dictionary; it's expecting keyword arguments. .format({'F': 4}) should be changed to .format(F=4). To do that with your dictionary, use **:

print(G3.format(**params))

For more information on argument unpacking, see the docs.

answered Mar 14, 2016 at 14:51

zondo

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2 Comments

RobbG Over a year ago

facepalm Thanks, I feel like a real idiot now looking about 4 lines above my code snippet and seeing exactly that where it's working properly.

zondo Over a year ago

Don't worry. It took me a minute to see your problem ;)

gtlambert · Accepted Answer · 2016-03-14 14:51:24Z

4

You just need to unpack the dictionary into the format string using the ** operator:

print(G3.format(**params))

Output

G3 F 250 I -22.5 J -22.5 X 7.4 Y 12.817175976 Z -50

answered Mar 14, 2016 at 14:51

gtlambert

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