2020-05-29 19:26:03 +05:30
|
|
|
[Contents](../Contents.md) \| [Previous (7.1 Variable Arguments)](01_Variable_arguments.md) \| [Next (7.3 Returning Functions)](03_Returning_functions.md)
|
2020-05-28 17:41:59 -05:00
|
|
|
|
2020-05-26 09:21:19 -05:00
|
|
|
# 7.2 Anonymous Functions and Lambda
|
|
|
|
|
|
|
|
|
|
### List Sorting Revisited
|
|
|
|
|
|
|
|
|
|
Lists can be sorted *in-place*. Using the `sort` method.
|
|
|
|
|
|
|
|
|
|
```python
|
|
|
|
|
s = [10,1,7,3]
|
|
|
|
|
s.sort() # s = [1,3,7,10]
|
|
|
|
|
```
|
|
|
|
|
|
|
|
|
|
You can sort in reverse order.
|
|
|
|
|
|
|
|
|
|
```python
|
|
|
|
|
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?
|
|
|
|
|
|
|
|
|
|
```python
|
|
|
|
|
[{'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?
|
|
|
|
|
|
2020-05-28 17:41:59 -05:00
|
|
|
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.
|
2020-05-26 09:21:19 -05:00
|
|
|
|
|
|
|
|
```python
|
|
|
|
|
def stock_name(s):
|
|
|
|
|
return s['name']
|
|
|
|
|
|
|
|
|
|
portfolio.sort(key=stock_name)
|
|
|
|
|
```
|
|
|
|
|
|
2020-05-28 17:41:59 -05:00
|
|
|
Here's the result.
|
2020-05-26 09:21:19 -05:00
|
|
|
|
|
|
|
|
```python
|
|
|
|
|
# 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
|
|
|
|
|
|
2020-05-28 17:41:59 -05:00
|
|
|
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.
|
2020-05-26 09:21:19 -05:00
|
|
|
|
|
|
|
|
### Lambda: Anonymous Functions
|
|
|
|
|
|
2020-05-28 17:41:59 -05:00
|
|
|
Use a lambda instead of creating the function. In our previous
|
|
|
|
|
sorting example.
|
2020-05-26 09:21:19 -05:00
|
|
|
|
|
|
|
|
```python
|
|
|
|
|
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.
|
|
|
|
|
|
|
|
|
|
```python
|
|
|
|
|
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:
|
|
|
|
|
|
|
|
|
|
```python
|
|
|
|
|
>>> 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)
|
|
|
|
|
>>>
|
|
|
|
|
```
|
|
|
|
|
|
2020-05-26 15:50:07 -05:00
|
|
|
### Exercise 7.5: Sorting on a field
|
2020-05-26 09:21:19 -05:00
|
|
|
|
|
|
|
|
Try the following statements which sort the portfolio data
|
|
|
|
|
alphabetically by stock name.
|
|
|
|
|
|
|
|
|
|
```python
|
|
|
|
|
>>> 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.
|
|
|
|
|
|
2020-05-26 15:50:07 -05:00
|
|
|
### Exercise 7.6: Sorting on a field with lambda
|
2020-05-26 09:21:19 -05:00
|
|
|
|
|
|
|
|
Try sorting the portfolio according the number of shares using a
|
|
|
|
|
`lambda` expression:
|
|
|
|
|
|
|
|
|
|
```python
|
|
|
|
|
>>> 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
|
|
|
|
|
|
|
|
|
|
```python
|
|
|
|
|
>>> 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
|
2020-05-28 17:41:59 -05:00
|
|
|
opposed to having to define a separate function first.
|
2020-05-26 09:21:19 -05:00
|
|
|
|
2020-05-29 19:26:03 +05:30
|
|
|
[Contents](../Contents.md) \| [Previous (7.1 Variable Arguments)](01_Variable_arguments.md) \| [Next (7.3 Returning Functions)](03_Returning_functions.md)
|
