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7.2 Anonymous Functions and Lambda
List Sorting Revisited
Lists can be sorted in-place. Using the sort
method.
s = [10,1,7,3]
s.sort() # s = [1,3,7,10]
You can sort in reverse order.
s = [10,1,7,3]
s.sort(reverse=True) # s = [10,7,3,1]
It seems simple enough. However, how do we sort a list of dicts?
[{'name': 'AA', 'price': 32.2, 'shares': 100},
{'name': 'IBM', 'price': 91.1, 'shares': 50},
{'name': 'CAT', 'price': 83.44, 'shares': 150},
{'name': 'MSFT', 'price': 51.23, 'shares': 200},
{'name': 'GE', 'price': 40.37, 'shares': 95},
{'name': 'MSFT', 'price': 65.1, 'shares': 50},
{'name': 'IBM', 'price': 70.44, 'shares': 100}]
By what criteria?
You can guide the sorting by using a key function. The key function is a function that receives the dictionary and returns the value of interest for sorting.
def stock_name(s):
return s['name']
portfolio.sort(key=stock_name)
Here's the result.
# Check how the dictionaries are sorted by the `name` key
[
{'name': 'AA', 'price': 32.2, 'shares': 100},
{'name': 'CAT', 'price': 83.44, 'shares': 150},
{'name': 'GE', 'price': 40.37, 'shares': 95},
{'name': 'IBM', 'price': 91.1, 'shares': 50},
{'name': 'IBM', 'price': 70.44, 'shares': 100},
{'name': 'MSFT', 'price': 51.23, 'shares': 200},
{'name': 'MSFT', 'price': 65.1, 'shares': 50}
]
Callback Functions
In the above example, the key function is an example of a callback
function. The sort()
method "calls back" to a function you supply.
Callback functions are often short one-line functions that are only
used for that one operation. Programmers often ask for a short-cut
for specifying this extra processing.
Lambda: Anonymous Functions
Use a lambda instead of creating the function. In our previous sorting example.
portfolio.sort(key=lambda s: s['name'])
This creates an unnamed function that evaluates a single expression. The above code is much shorter than the initial code.
def stock_name(s):
return s['name']
portfolio.sort(key=stock_name)
# vs lambda
portfolio.sort(key=lambda s: s['name'])
Using lambda
- lambda is highly restricted.
- Only a single expression is allowed.
- No statements like
if
,while
, etc. - Most common use is with functions like
sort()
.
Exercises
Read some stock portfolio data and convert it into a list:
>>> import report
>>> portfolio = list(report.read_portfolio('Data/portfolio.csv'))
>>> for s in portfolio:
print(s)
Stock('AA', 100, 32.2)
Stock('IBM', 50, 91.1)
Stock('CAT', 150, 83.44)
Stock('MSFT', 200, 51.23)
Stock('GE', 95, 40.37)
Stock('MSFT', 50, 65.1)
Stock('IBM', 100, 70.44)
>>>
Exercise 7.5: Sorting on a field
Try the following statements which sort the portfolio data alphabetically by stock name.
>>> def stock_name(s):
return s.name
>>> portfolio.sort(key=stock_name)
>>> for s in portfolio:
print(s)
... inspect the result ...
>>>
In this part, the stock_name()
function extracts the name of a stock from
a single entry in the portfolio
list. sort()
uses the result of
this function to do the comparison.
Exercise 7.6: Sorting on a field with lambda
Try sorting the portfolio according the number of shares using a
lambda
expression:
>>> portfolio.sort(key=lambda s: s.shares)
>>> for s in portfolio:
print(s)
... inspect the result ...
>>>
Try sorting the portfolio according to the price of each stock
>>> portfolio.sort(key=lambda s: s.price)
>>> for s in portfolio:
print(s)
... inspect the result ...
>>>
Note: lambda
is a useful shortcut because it allows you to
define a special processing function directly in the call to sort()
as
opposed to having to define a separate function first.
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