# 1. Function Basics

## 1.1. Function definition

• Reuse code

• Clean-up code

• Allows for easier refactoring

```def hello():
print('My name... José Jiménez')

hello()     # My name... José Jiménez
hello()     # My name... José Jiménez
hello()     # My name... José Jiménez
```

## 1.2. Returning values

• `return` keyword indicates outcome of the function

• Code after `return` will not execute

```def hello():
return 'ehlo world'

output = hello()

print(output)
# 'ehlo world'
```
```def hello():
return 'ehlo world'
print('This will not be executed')

output = hello()

print(output)
# 'ehlo world'
```

### 1.2.1. Returning simple types

```def function():
return 42
```
```def function():
return 13.37
```
```def function():
return 'Mark Watney'
```
```def function():
return (42, 13.37, 'Mark Watney')
```
```def function():
return 42, 13.37, 'Mark Watney'
```
```def function():
return [42, 13.37, 'Mark Watney']
```
```def function():
return {42, 13.37, 'Mark Watney'}
```
```def function():
return {'first_name': 'Mark', 'last_name': 'Watney'}
```
```def function():
return True
```
```def function():
return None
```
```def function():
print('ehlo world')
# Python will ``return None`` if no explicit return is specified
```
```def function():
pass
# Python will ``return None`` if no explicit return is specified
```
```def function():
# Python will ``return None`` if no explicit return is specified
```

### 1.2.2. Returning nested types

```def function():
return [
('Mark', 'Watney'),
{'Jan Twardowski', 'Melissa Lewis'},
{'astro': 'Иванович', 'agency': {'name': 'Roscosmos'}},
{'astro': 'Jiménez', 'missions': ('Mercury', 'Gemini', 'Apollo')},
]
```

## 1.3. Function arguments

### 1.3.1. Required arguments

```def subtract(a, b):
return a - b
```

### 1.3.2. Arguments with default value

• Arguments without default values are required

• Function will take default value if not overwritten by user

• Arguments with default values must be at the right side

• Arguments with default values can be omitted while executing

```def subtract(a=1, b=2):
return a - b
```
```def subtract(a, b=2):
return a - b
```
```def subtract(a=1, b):
return a - b

# SyntaxError: non-default argument follows default argument
```

### 1.3.3. Positional arguments

```def subtract(a, b):
return a - b

subtract(2, 1)      # 1
subtract(1, 2)      # -1
```

### 1.3.4. Keyword arguments

• Arguments without default values are required

• Order of keyword arguments has no significance

```def subtract(a, b):
return a - b

subtract(a=2, b=1)  # 1
subtract(b=1, a=2)  # 1
subtract(2, b=1)    # 1
subtract(a=2, 1)    # SyntaxError: positional argument follows keyword argument
```
```def hello(name='José Jiménez'):
print(f'My name... {name}')

hello('Mark Watney')          # My name... Mark Watney
hello(name='Mark Watney')     # My name... Mark Watney
hello()                       # My name... José Jiménez
```
```def connect(username, password, host='127.0.0.1',
port=80, ssl=True, keep_alive=1, persistent=False):
print('Connecting...')

connect(
host='localhost',
port=443,
ssl=True,
persistent=True,
)
```
```def read_csv(filepath_or_buffer, sep=', ', delimiter=None, header='infer',
names=None, index_col=None, usecols=None, squeeze=False, prefix=None,
mangle_dupe_cols=True, dtype=None, engine=None, converters=None,
true_values=None, false_values=None, skipinitialspace=False,
skiprows=None, nrows=None, na_values=None, keep_default_na=True,
na_filter=True, verbose=False, skip_blank_lines=True, parse_dates=False,
infer_datetime_format=False, keep_date_col=False, date_parser=None,
dayfirst=False, iterator=False, chunksize=None, compression='infer',
thousands=None, decimal=b'.', lineterminator=None, quotechar='"',
quoting=0, escapechar=None, comment=None, encoding=None, dialect=None,
skipfooter=0, doublequote=True, delim_whitespace=False, low_memory=True,
memory_map=False, float_precision=None):
"""
"""

filepath_or_buffer='iris.csv',
encoding='utf-8',
verbose=True,
usecols=['Sepal Length', 'Species']
)
```

## 1.4. Naming convention

### 1.4.1. Function name convention

• It's not Java, do not use `camelCase`

```def addNumbers(a, b):
return a + b
```
• It's Python, use `snake_case` # Python - snake ;)

```def add_numbers(a, b):
return a + b
```

### 1.4.2. Use better names, rather than comments

```def cal_var(results):
"""Calculate variance"""
return sum((Xi-m) ** 2 for Xi in results) / len(results)
```
```def calculate_variance(results):
return sum((Xi-m) ** 2 for Xi in results) / len(results)
```

### 1.4.3. Name collisions

• `_` at the end of name when name collision

```def print_(text):
print(f'<strong>{text}</strong>')
```

### 1.4.4. System functions names

• `__` at the beginning and end of name

```def __import__(module_name):
...
```

## 1.5. Variable scope

### 1.5.1. Global scope

• All variables in main program

• Variables are available inside all functions

```print(globals())
# {...}
```

### 1.5.2. Local scope

• Variables defined inside function

• Variables are not available from outside

```print(locals())
# {...}
```
```def add_numbers(a, b=2):
c = 3
print(locals())

# {'a': 1, 'b': 2, 'c': 3}
```

## 1.6. Assignments

### 1.6.1. Cleaning text input

• Complexity level: easy

• Lines of code to write: 15 lines

• Estimated time of completion: 15 min

English
1. For given input data (see below)

2. Write function cleaning up data

Polish
1. Dla danych wejściowych (patrz poniżej)

2. Napisz funkcję czyszczącą dane

Input
```'ul.Mieszka II'
'UL. Zygmunta III WaZY'
'  bolesława chrobrego '
'ul Jana III SobIESkiego'
'\tul. Jana trzeciego Sobieskiego'
'ulicaJana III Sobieskiego'
'UL. JA\tNA 3 SOBIES  KIEGO'
'ULICA JANA III SOBIESKIEGO  '
'ULICA. JANA III SOBIeskieGO'
' Jana 3 Sobieskiego  '
'Jana III Sobi\teskiego '
```
Output
```'Mieszka II'
'Zygmunta III Wazy'
'Bolesława Chrobrego'
'Jana III Sobieskiego'
'Jana III Sobieskiego'
'Jana III Sobieskiego'
'Jana III Sobieskiego'
'Jana III Sobieskiego'
'Jana III Sobieskiego'
'Jana III Sobieskiego'
'Jana III Sobieskiego'
```
The whys and wherefores
• Defining and calling functions

• Passing function arguments

• Cleaning data from user input

Todo

Translate input data to English

### 1.6.2. Aviation numbers

• Complexity level: easy

• Lines of code to write: 15 lines

• Estimated time of completion: 15 min

English
1. For input data (see below)

2. Define function converting `int` or `float` to text form in Pilot's Speak

Polish
1. Dla danych wejściowych (patrz poniżej)

2. Zdefiniuj funkcję konwertującą `int` lub `float` na formę tekstową w mowie pilotów

Input
```0, "zero"
1, "one"
2, "two"
3, "tree"
4, "fower"
5, "fife"
6, "six"
7, "seven"
8, "ait"
9, "niner"
```
```1969
31337
13.37
31.337
-1969
-31.337
-49.35
```
Output
```'one niner six niner'
'tree one tree tree seven'
'one tree and tree seven'
'tree one and tree tree seven'
'minus one niner six niner'
'minus tree one and tree tree seven'
'minus fower niner and tree fife'
```
The whys and wherefores
• Defining and calling functions

• Passing function arguments

• Cleaning data from user input

• `dict` lookups

### 1.6.3. Number to human readable

• Complexity level: medium

• Lines of code to write: 15 lines

• Estimated time of completion: 15 min

English
1. For input data (see below)

2. Define function converting `int` or `float` to text form

3. Text form must be in proper grammar form

4. Max 6 digits before decimal separator (point `.`)

5. Max 5 digits after decimal separator (point `.`)

Polish
1. Dla danych wejściowych (patrz poniżej)

2. Zdefiniuj funkcję konwertującą `int` lub `float` na formę tekstową

3. Forma tekstowa musi być poprawna gramatycznie

4. Max 6 cyfr przed separatorem dziesiętnym (point `.`)

5. Max 5 cyfr po separatorze dziesiętnym (point `.`)

Input
```1969
31337
13.37
31.337
-1969
-31.337
-49.35
```
Output
```'one thousand nine hundred sixty nine'
'thirty one thousand three hundred thirty seven'
'thirteen and thirty seven hundredths'
'thirty one three hundreds thirty seven thousands'
'minus one thousand nine hundred sixty nine'
'minus thirty one three hundreds thirty seven thousands'
'minus forty nine and thirty five hundreds'
```
The whys and wherefores
• Defining and calling functions

• Passing function arguments

• Cleaning data from user input

• `dict` lookups

### 1.6.4. Roman numbers

• Complexity level: medium

• Lines of code to write: 15 lines

• Estimated time of completion: 15 min

English
1. Define function converting roman numerals to integer

2. Define function converting integer to roman numerals

Polish
1. Zdefiniuj funkcję przeliczającą liczbę rzymską na całkowitą

2. Zdefiniuj funkcję przeliczającą liczbę całkowitą na rzymską

The whys and wherefores
• Defining and calling functions

• Checking for corner cases

• Passing function arguments

• Cleaning data from user input

• `dict` lookups