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Notes/07_Advanced_Topics/02_Anonymous_function.md

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+# 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?
+
+You can guide the sorting by using a *key function*. The *key function* is a function that receives the dictionary and returns the value in a specific key.
+
+```python
+def stock_name(s):
+    return s['name']
+
+portfolio.sort(key=stock_name)
+```
+
+The value returned by the *key function* determines the sorting.
+
+```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
+
+Callback functions are often short one-line functions that are only used for that one operation. For example of previous sorting example.
+Programmers often ask for a short-cut, so is there a shorter way to specify custom processing for `sort()`?
+
+### Lambda: Anonymous Functions
+
+Use a lambda instead of creating the function.
+In our previous sorting example.
+
+```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)
+>>>
+```
+
+### (a) Sorting on a field
+
+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.
+
+### (b) Sorting on a field with lambda
+
+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
+opposed to having to define a separate function first (as in part a).
+
+[Next](03_Returning_functions)