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Merged
Scottan merged 8 commits into from
Apr 13, 2026
Merged

21 make errors example into a function #56

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9817f1d
Replace test code with a function to avoid repetition
Scottan Nov 14, 2025
8126441
Enhance explanation of try-except usage in Python
Scottan Nov 14, 2025
34c7fcc
Fix EAFP link formatting in defensive programming section
Scottan Nov 14, 2025
609571e
broke up paragraph
Scottan Nov 14, 2025
bcd4a7c
fix: replaced check_value with check_sign function
Apr 13, 2026
2f2b7d0
style: changed else clause in try/except/else/finally example
Apr 13, 2026
d1be0af
feat: better try/except/else case
Apr 13, 2026
868162d
style: deleted some unnecesary print statements to shorten example
Apr 13, 2026
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3 changes: 3 additions & 0 deletions .gitignore
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Original file line number Diff line number Diff line change
Expand Up @@ -52,3 +52,6 @@ vendor/
.docker-vendor/
Gemfile.lock
.*history

# Course developer file to testing lesson code
test.ipynb
108 changes: 45 additions & 63 deletions episodes/05-defensive_programming.md
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Expand Up @@ -30,15 +30,15 @@ More importantly, applications are not expected to crash and we are going to lea

Please look at the following code. Can you find the fundamental problem in this test?

```bash
```python
val = 1

if val>0 and val<10:
print('Value: ', val, 'is a digit.')
elif val==0:
print('Value ', val, 'is nul')
if val > 0:
print('Value:', val, 'is positive.')
elif val == 0:
print('Value:', val, 'is zero.')
else:
print('Value: ', val, 'is a number.')
print('Value:', val, 'is negative.')
```

::::::::::::::: solution
Expand All @@ -47,26 +47,26 @@ else:

The test assumes that `val` is a number, and throws an uncontrolled error if it is not.

```bash
```python
val = 'a'

if val>0 and val<10:
print('Value: ', val, 'is a digit.')
elif val==0:
print('Value ', val, 'is nul')
if val > 0:
print('Value:', val, 'is positive.')
elif val == 0:
print('Value:', val, 'is zero.')
else:
print('Value: ', val, 'is a number.')
print('Value:', val, 'is negative.')
```

```output
---------------------------------------------------------------------------
TypeError Traceback (most recent call last)
<ipython-input-2-99c0e25bf5e9> in <module>()
1 val = 'a'
2
----> 3 if val>0 and val<10:
4 print('Value: ', val, 'is a digit.')
5 elif val==0:
1 def check_sign(val):
----> 2 if val > 0:
3 print('Value:', val, 'is positive.')
4 elif val == 0:
5 print('Value:', val, 'is zero.')

TypeError: '>' not supported between instances of 'str' and 'int'
```
Expand All @@ -75,16 +75,24 @@ TypeError: '>' not supported between instances of 'str' and 'int'

::::::::::::::::::::::::::::::::::::::::::::::::::

We can avoid problems like this by wrapping our code in an `if` statement:
We can avoid problems like this by wrapping our code in an `if` statement.
To make things simpler, we will first write the test as a function:

```python
if type(val) is int or type(val) is float:
if val>0 and val<10:
print('Value: ', val, 'is a digit.')
elif val==0:
print('Value ', val, 'is nul')
def check_sign(val):
if val > 0:
print('Value:', val, 'is positive.')
elif val == 0:
print('Value:', val, 'is zero.')
else:
print('Value: ', val, 'is a number.')
print('Value:', val, 'is negative.')
```

Then wrap the function call in an `if` statement:

```python
if type(val) is int or type(val) is float:
check_sign(val)
else:
print('val is not a number')
```
Expand All @@ -95,29 +103,21 @@ Python provides the `try-except` structure to avoid this issue, enabling develop

```python
try:
if val>0 and val<10:
print('Value: ', val, 'is a digit.')
elif val==0:
print('Value ', val, 'is nul')
else:
print('Value: ', val, 'is a number.')
check_sign(val)
except:
print('Val is not a number')
print('Enter a new number')
```

At the top of the statement is the code that we are interested in executing, which is run in the `try` statement. If that fails then the `except` statement comes into effect, (hopefully) returning helpful information to the user about what happened and giving them some guidance on how to avoid the problem in future.

Using `try-except` statements results in clearer, easier to understand code by following the common Python coding style of [EAFP](https://docs.python.org/3.6/glossary.html#term-eafp) (it's easier to ask for forgiveness than permission). This style shows the code we want to execute first, assuming that the incoming data is correct, before dealing with exceptions if the assumptions prove false.

The `except` statement will catch all errors and so we do not, initially at least, need to know exactly what errors we are trying to avoid. However, python does provide error codes, which we can use to expand the structure to capture specific error types. For the example above, we would want to capture a `TypeError`:

```python
try:
if val>0 and val<10:
print('Value: ', val, 'is a digit.')
elif val==0:
print('Value ', val, 'is nul')
else:
print('Value: ', val, 'is a number.')
check_sign(val)
except TypeError as err:
print('Val is not a number')
print('But our code does not crash anymore')
Expand All @@ -130,21 +130,18 @@ As with `if` statements, multiple `except` statements can be used, each with a d

```python
try:
if val>0 and val<10:
print('Value: ', val, 'is a digit.')
elif val==0:
print('Value ', val, 'is nul')
else:
print('Value: ', val, 'is a number.')
check_sign(val)
reciprocal = 1/val
except TypeError as err:
print('Val is not a number')
print('But our code does not crash anymore')
print('The run-time error is:', err)
except Exception as err:
print('Some error other than a TypeError occured')
print('The run-time error is:', err)
else:
print('1/val = ', 1/val)
print('The reciprocal of the value =', reciprocal)
finally:
print('release memory')
del(val)
```

The typical use of the `finally` statement is to deal with the release of external resources (such as files or network connections) whether or not the attempted action has been successful.
Expand Down Expand Up @@ -219,12 +216,7 @@ val = 'a'

assert type(val) is float or type(val) is int, "Variable has to be a numerical object"

if val>0 and val<10:
print('Value: ', val, 'is a digit.')
elif val==0:
print('Value ', val, 'is nul')
else:
print('Value: ', val, 'is a number.')
check_sign(val)
```

```output
Expand Down Expand Up @@ -252,12 +244,7 @@ val = np.nan

assert type(val) is float or type(val) is int, "Variable has to be a numerical object"

if val>0 and val<10:
print('Value: ', val, 'is a digit.')
elif val==0:
print('Value ', val, 'is nul')
else:
print('Value: ', val, 'is a number.')
check_sign(val)
```

```output
Expand All @@ -281,12 +268,7 @@ val = np.nan
assert type(val) is float or type(val) is int, "Variable has to be a numerical object"
assert not np.isnan(val), "Variable must not be a NaN"

if val>0 and val<10:
print('Value: ', val, 'is a digit.')
elif val==0:
print('Value ', val, 'is nul')
else:
print('Value: ', val, 'is a number.')
check_sign(val)
```

```output
Expand Down
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