Imagine that you are developing software for the big shipping company(why would you imagine small anyway). And you got a task to create a function for calculating a charge for ships based on their cargo weight. Easy breezy:

```
WEIGHT_RATES = [
( 10, 10.55),
( 5, 5.05),
( 2, 3.35),
( 0, 1.25)
]
def shipping_charge(weight):
if weight < 0:
raise ValueError("Can't calculate shipping charge of negative weights")
for min_weight, rate in WEIGHT_RATES:
if weight > min_weight:
return weight * rate
```

Simple enough.

But then one day your program eventually is going to work in another country, say the USA. One problem arises: we need to use pounds instead of kilograms for charging. No problem, here you are:

```
def shipping_charge(weight, pnds):
if pnds:
weight /= 2.2
if weight < 0:
raise ValueError("Can't calculate shipping charge of negative weights")
for min_weight, rate in WEIGHT_RATES:
if weight > min_weight:
return weight * rate
```

This is getting complicated, but then one more requirement - you need to raise an exception if weight exceeds 1000 kilograms for specific directions:

```
def shipping_charge(weight, pnds, exceed):
if pnds:
weight /= 2.2
if exceed and weight > 1000:
raise Exception("Weight can't exceed 1000 kg")
if weight < 0:
raise ValueError("Can't calculate shipping charge of negative weights")
for min_weight, rate in WEIGHT_RATES:
if weight > min_weight:
return weight * rate
```

You see the problem? On this stupid example, you came up to function with 3 positional arguments, two last of them with the same type. The end-user, or you as a developer can easily forget which one needs to come first and messed them up. Due to the same type Python program will not fail and you will get a logic error:

`shipping_charge(2000, True, False)`

or

`shipping_charge(2000, False, True)`

You can use a keyword arguments with a default values and it's a good practice:

```
def shipping_charge(weight, pnds=False, exceed=False):
if pnds:
weight /= 2.2
if exceed and weight > 1000:
raise Exception("Weight can't exceed 1000 kg")
if weight < 0:
raise ValueError("Can't calculate shipping charge of negative weights")
for min_weight, rate in WEIGHT_RATES:
if weight > min_weight:
return weight * rate
```

But the problem is not solved. To solve the problem you need to add one star in the argument list:

```
def shipping_charge(weight, *, pnds=False, exceed=False):
if pnds:
weight /= 2.2
if exceed and weight > 1000:
raise Exception("Weight can't exceed 1000 kg")
if weight < 0:
raise ValueError("Can't calculate shipping charge of negative weights")
for min_weight, rate in WEIGHT_RATES:
if weight > min_weight:
return weight * rate
```

That's it, next time you will call this function you will got an error:

```
>>>shipping_charge(2000, True, False)
>>>
TypeError: shipping_charge() takes 1 positional argument but 3 were given
```

More info:PEP-3102