Why does input() activate when not being called in dictionary?-CodePudding

In my code, I have a dictionary of values along with a key called 'custom' which allows me to input an arbitrary value. When I print key values that are NOT 'custom' it still hits the input() field:

Ip = {'argon':15.76,
      'neon':21.56,
      'krypton':14.00,
      'xenon':12.13,
      'helium':24.59,
      'hydrogen':15.42,
      'custom': float(input('Enter ionization potential (eV): '))}

print(Ip['argon'], Ip['helium'])

output:

Enter ionization potential (eV): 1
15.76 24.59

I was under the impression that keys and their associated values are kept suppressed until called, but clearly I misunderstand how dictionaries work.

A quick and dirty way of ameliorating this was to make a conditional statement, but there could be (for some reason) multiple input() dictionary definitions such that I'd have to hard-code a conditional statement for each one. Is there a more Pythonic way of doing this?

def func(key):
    
    if key == 'custom':
        Ip = {key:float(input('Enter ionization potential (eV): '))}

# Is there a way to remove the need for a conditional statement?
    else:
        Ip = {'argon':15.76,
              'neon':21.56,
              'krypton':14.00,
              'xenon':12.13,
              'helium':24.59,
              'hydrogen':15.42}
        
    return print(Ip[key])

func('argon')

output:

15.76

CodePudding user response：

It is being called, as you've realized. When you define the dict, the call is evaluated and its result is inserted into the dict under 'custom', so in that example, 'custom': 1.0. To avoid the call, make it a function instead.

Now that raises the question of how to have mixed types in a dict (floats and functions) and special-casing the functions. One way to get around that is to have two separate dicts: one with floats and a fallback one with functions. For example:

def func(key):
    ionization_potential = {
        'argon': 15.76,
        'neon': 21.56,
        'krypton': 14.00,
        'xenon': 12.13,
        'helium': 24.59,
        'hydrogen': 15.42}
    functions = {
        'custom': lambda: float(input('Enter ionization potential (eV): '))}
    try:
        v = ionization_potential[key]
    except KeyError:
        f = functions[key]
        v = f()
    return v

Demo:

>>> func('argon')
15.76
>>> func('custom')
Enter ionization potential (eV): 1
1.0

Lastly, if a key is not found in either dict, this will raise a KeyError from KeyError, so if I were you, I'd raise a ValueError from None in that case, since the problem is with the value of the argument:

try:
    f = functions[key]
except KeyError:
    raise ValueError(f'key not found: {key!r}') from None
v = f()

Demo:

>>> func('foobar')
Traceback (most recent call last):
  ...
ValueError: key not found: 'foobar'