How to understand code of "Exercise 40: Dictionaries .." in Learn Python The Hard Way
Question:
I’m trying to follow along with Exercise 40 from Learn Python The Hard Way, 2nd edition", by Zed A. Shaw.
The last snippet of code sets up two dictionaries like so:
states = {
'Oregon': 'OR',
'Florida': 'FL',
'California': 'CA',
'New York': 'NY',
'Michigan': 'MI'
}
cities = {
'CA': 'San Francisco',
'MI': 'Detroit',
'FL': 'Jacksonville'
}
and then does various things with the dictionary.
I don’t understand what is happening at this point in the example:
print "Michigan has: ", cities[states['Michigan']]
When I try it, the result is Michigan has: Detroit.
Why? How does Python link one dictionary with the other?
This is my assumption and understanding I want to verify:
Python reads cities[states['Michigan']] backwards. First, it looks through the states and finds Michigan, then its value. Next it uses this value to look in cities, where the value from the state is the key. Finally it prints out the value for the key from cities.
Why does it not just print out the key from the cities (MI)?
Answers:
Python does not link dictionaries. What it does is in line cities[states['Michigan']] it first evaluates states['Michigan'] to the value of 'Michigan' in states dictionary (which is 'MI'). After that expression looks like cities['MI']. And then it evaluates it to value of 'MI' from cities dictionary (which is 'Detroit').
It resolves not quite backwards, but in order of innermost expression -> outermost expression.
So, for cities[states['Michigan']], it will
- Looks at
cities
- Recognize it needs to resolve
states['Michigan']
- Look at
states
- Find
states['Michigan']
- Return value of
states['Michigan'] to cities
- Now it can resolve
cities[states['Michigan']]
This is how the Python interpreter evaluates the expressions for a dictionary lookup.
-
In following expression it looks for the value of given key 'Michigan' in the dict states:
states['Michigan']
The resulting value, after evaluating this expression, is "MI".
-
Then it takes this evaluated value and uses it to lookup inside the cities dict. Knowing the first expression returned "MI" an equivalent lookup expression would be:
cities['MI']
I’m trying to follow along with Exercise 40 from Learn Python The Hard Way, 2nd edition", by Zed A. Shaw.
The last snippet of code sets up two dictionaries like so:
states = {
'Oregon': 'OR',
'Florida': 'FL',
'California': 'CA',
'New York': 'NY',
'Michigan': 'MI'
}
cities = {
'CA': 'San Francisco',
'MI': 'Detroit',
'FL': 'Jacksonville'
}
and then does various things with the dictionary.
I don’t understand what is happening at this point in the example:
print "Michigan has: ", cities[states['Michigan']]
When I try it, the result is Michigan has: Detroit.
Why? How does Python link one dictionary with the other?
This is my assumption and understanding I want to verify:
Python reads cities[states['Michigan']] backwards. First, it looks through the states and finds Michigan, then its value. Next it uses this value to look in cities, where the value from the state is the key. Finally it prints out the value for the key from cities.
Why does it not just print out the key from the cities (MI)?
Python does not link dictionaries. What it does is in line cities[states['Michigan']] it first evaluates states['Michigan'] to the value of 'Michigan' in states dictionary (which is 'MI'). After that expression looks like cities['MI']. And then it evaluates it to value of 'MI' from cities dictionary (which is 'Detroit').
It resolves not quite backwards, but in order of innermost expression -> outermost expression.
So, for cities[states['Michigan']], it will
- Looks at
cities - Recognize it needs to resolve
states['Michigan'] - Look at
states - Find
states['Michigan'] - Return value of
states['Michigan']tocities - Now it can resolve
cities[states['Michigan']]
This is how the Python interpreter evaluates the expressions for a dictionary lookup.
-
In following expression it looks for the value of given key
'Michigan'in the dictstates:states['Michigan']The resulting value, after evaluating this expression, is
"MI". -
Then it takes this evaluated value and uses it to lookup inside the
citiesdict. Knowing the first expression returned"MI"an equivalent lookup expression would be:cities['MI']