how to build a Python dict value from another one during dict creation?

Question

I would like to build a dict in which one value is built from another.

I thought writing

d = {
    'a':1,
    'b':self['a']+1
}

but it did not work as expected :

>>> {'a':1, 'b':self['a']+1}
Traceback (most recent call last):
  File "<stdin>", line 1, in ?
NameError: name 'self' is not defined

So, how can I achieve that Python trick ? (I'm using Python 2.4 )

Answer 1

You can't reference a dictionary that has not yet been created. Just assign additional keys after creating the dictionary:

d = {'a': 1}
d['b'] = d['a'] + 1

Alternatively, set the value to a separate variable first , then create the dictionary:

a_value = 1
d = {'a': a_value, 'b': a_value + 1}

Answer 2

If you only have two values, calculate a first, then add b as a separate line.

If the dependencies are more complex, it may be worthwhile to use a function to provide values.

Python 2.7 onwards has dictionary comprehension:

mydict = {key: value for (key, value) in iterable}

Equivalent in earlier would be:

mydict = dict((key, value) for (key, value) in iterable)

Now you provide the iterable function for generating the values:

def get_dictionary_values():
    a = get_really_expensive_calculation()
    yield ('a', a)
    b = a + modification_value
    yield ('b', b)
    # Add all other values here that calculate from earlier values.

mydict = dict((key, value) for (key, value) in get_dictionary_values())

With options of loops inside the function as needed.