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?

Answer 1

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.