A KeyError is generated when trying to access a key that does not exist in the dictionary.
The countermeasures against this are described below.
>>> pythons = {'Chapman': 'Graham', 'Cleese': 'John',
... Jones': 'Terry', 'Palin': 'Michael'}
>>> 'Marx' in pythons
False
>>> pythons.get('Cleese')
'John'
If the key does not exist, the specified option value is returned.
>>> pythons.get('Marx', 'Not a Python')
'Not a Python'
If the key does not exist, None is returned if no option is specified.
>>> ret = pythons.get('Marx')
>>> ret is None
True
If the key is not in the dictionary, it will be added to the dictionary with the value specified in the second argument.
>>> periodic_table = {'Hydrogen', 1, 'Helium': 2}
>>> print(periodic_table)
{'Helium': 2, 'Hydrogen': 1}
>>>
carbon = periodic_table.setdefault ('Carbon', 12) # Added because there is no'Carbon' in the dictionary 12
periodic_table {'Helium': 2, 'Carbon': 12, 'Hydrogen': 1}
If the key is already in the dictionary, the original value is returned and the dictionary is not changed at all.
>>> helium = periodic_table.setdefault('Helium', 947)
>>> helium
>>> periodic_table
{'Helium': 2, 'Carbon': 12, 'Hydrogen': 1}
defaultdict ()
sets the default value in advance when creating a dictionary.
Specify a function in the argument of defaultdict ()
.
from collections import defaultdict
periodic_table = defaultdict(int)
Then, if you access a key that does not exist, the value of ʻint () `(= 0) will be set.
>>> periodic_table['Hydrogen'] = 1
>>> periodic_table['Lead']
0
>>> periodic_table
defaultdict(<class 'int'>, {'Lead': 0, 'Hydrogen': 1})
The function passed to defaultdict ()
may be your own definition.
In this case, the value of no_idea ()
will be set.
>>> from collections import defaultdict
>>>
>>> def no_idea():
... return 'Huh?'
...
>>> bestiary = defaultdict(no_idea)
>>> bestiary['A'] = 'Abominable Snowman'
>>> bestiary['B'] = 'Basilisk'
>>> bestiary['A']
'Adominable Snowman'
>>> bestiary['B']
'Basilisk'
>>> bestiary['C']
'Huh?'
You can use lambda to define a default creation function in the defaultdict ()
call.
>>> bestiary = defaultdict(lambda: 'Huh?')
>>> bestiary = ['E']
'Huh?'
This code was borrowed from the following documents (partially modified).
You pointed out in the comments. You can also catch the exception.
try:
result = cache[x]
except KeyError:
result = 'blah blah blah...'
cache[x] = result
Recommended Posts