From ee641c189f124c02495de6cf38b863a36e427988 Mon Sep 17 00:00:00 2001
From: adii1823 <64855277+adii1823@users.noreply.github.com>
Date: Thu, 28 Oct 2021 17:37:54 +0530
Subject: [PATCH] ch04

---
 ch04/arguments.combined.py                    |  8 +++
 ch04/arguments.keyword.py                     |  5 ++
 ch04/arguments.multiple.value.py              | 13 +++++
 ch04/arguments.positional.keyword.py          | 15 +++++
 ch04/arguments.positional.py                  |  5 ++
 ch04/arguments.unpack.dict.py                 |  6 ++
 ch04/arguments.unpack.iterable.py             |  6 ++
 ch04/data.science.example.py                  | 22 +++++++
 ch04/docstrings.py                            | 24 ++++++++
 ch04/filter.lambda.py                         |  6 ++
 ch04/filter.regular.py                        | 10 ++++
 ch04/func.attributes.py                       | 16 +++++
 ch04/func_from.py                             |  5 ++
 ch04/func_import.py                           |  6 ++
 ch04/imports.py                               | 11 ++++
 ch04/key.points.argument.passing.py           |  6 ++
 ch04/key.points.assignment.py                 |  7 +++
 ch04/key.points.mutable.assignment.py         |  8 +++
 ch04/key.points.mutable.py                    |  7 +++
 ch04/lambda.explained.py                      | 30 ++++++++++
 ch04/lib/__init__.py                          |  0
 ch04/lib/funcdef.py                           |  6 ++
 ch04/matrix.multiplication.func.py            | 12 ++++
 ch04/matrix.multiplication.nofunc.py          |  7 +++
 ch04/no.side.effects.py                       | 10 ++++
 ch04/parameters.all.pkwonly.py                |  8 +++
 ch04/parameters.all.py                        |  7 +++
 ch04/parameters.default.py                    |  8 +++
 ...ters.defaults.mutable.intermediate.call.py | 11 ++++
 ch04/parameters.defaults.mutable.no.trap.py   |  5 ++
 ch04/parameters.defaults.mutable.py           | 11 ++++
 ch04/parameters.keyword.only.py               | 16 +++++
 ch04/parameters.positional.only.optional.py   |  6 ++
 ch04/parameters.positional.only.py            | 58 +++++++++++++++++++
 ch04/parameters.variable.db.py                | 15 +++++
 ch04/parameters.variable.keyword.py           |  7 +++
 ch04/parameters.variable.positional.py        | 12 ++++
 ch04/primes.py                                | 47 +++++++++++++++
 ch04/recursive.factorial.py                   |  8 +++
 ch04/return.multiple.py                       | 26 +++++++++
 ch04/return.none.py                           |  7 +++
 ch04/return.single.value.2.py                 | 12 ++++
 ch04/return.single.value.py                   | 11 ++++
 ch04/scoping.level.1.py                       |  9 +++
 ch04/scoping.level.2.global.py                | 16 +++++
 ch04/scoping.level.2.nonlocal.py              | 16 +++++
 ch04/scoping.level.2.py                       | 15 +++++
 ch04/vat.function.py                          |  7 +++
 ch04/vat.py                                   | 12 ++++
 49 files changed, 601 insertions(+)
 create mode 100644 ch04/arguments.combined.py
 create mode 100644 ch04/arguments.keyword.py
 create mode 100644 ch04/arguments.multiple.value.py
 create mode 100644 ch04/arguments.positional.keyword.py
 create mode 100644 ch04/arguments.positional.py
 create mode 100644 ch04/arguments.unpack.dict.py
 create mode 100644 ch04/arguments.unpack.iterable.py
 create mode 100644 ch04/data.science.example.py
 create mode 100644 ch04/docstrings.py
 create mode 100644 ch04/filter.lambda.py
 create mode 100644 ch04/filter.regular.py
 create mode 100644 ch04/func.attributes.py
 create mode 100644 ch04/func_from.py
 create mode 100644 ch04/func_import.py
 create mode 100644 ch04/imports.py
 create mode 100644 ch04/key.points.argument.passing.py
 create mode 100644 ch04/key.points.assignment.py
 create mode 100644 ch04/key.points.mutable.assignment.py
 create mode 100644 ch04/key.points.mutable.py
 create mode 100644 ch04/lambda.explained.py
 create mode 100644 ch04/lib/__init__.py
 create mode 100644 ch04/lib/funcdef.py
 create mode 100644 ch04/matrix.multiplication.func.py
 create mode 100644 ch04/matrix.multiplication.nofunc.py
 create mode 100644 ch04/no.side.effects.py
 create mode 100644 ch04/parameters.all.pkwonly.py
 create mode 100644 ch04/parameters.all.py
 create mode 100644 ch04/parameters.default.py
 create mode 100644 ch04/parameters.defaults.mutable.intermediate.call.py
 create mode 100644 ch04/parameters.defaults.mutable.no.trap.py
 create mode 100644 ch04/parameters.defaults.mutable.py
 create mode 100644 ch04/parameters.keyword.only.py
 create mode 100644 ch04/parameters.positional.only.optional.py
 create mode 100644 ch04/parameters.positional.only.py
 create mode 100644 ch04/parameters.variable.db.py
 create mode 100644 ch04/parameters.variable.keyword.py
 create mode 100644 ch04/parameters.variable.positional.py
 create mode 100644 ch04/primes.py
 create mode 100644 ch04/recursive.factorial.py
 create mode 100644 ch04/return.multiple.py
 create mode 100644 ch04/return.none.py
 create mode 100644 ch04/return.single.value.2.py
 create mode 100644 ch04/return.single.value.py
 create mode 100644 ch04/scoping.level.1.py
 create mode 100644 ch04/scoping.level.2.global.py
 create mode 100644 ch04/scoping.level.2.nonlocal.py
 create mode 100644 ch04/scoping.level.2.py
 create mode 100644 ch04/vat.function.py
 create mode 100644 ch04/vat.py

diff --git a/ch04/arguments.combined.py b/ch04/arguments.combined.py
new file mode 100644
index 0000000..c856aa7
--- /dev/null
+++ b/ch04/arguments.combined.py
@@ -0,0 +1,8 @@
+# arguments.combined.py
+def func(a, b, c, d, e, f):
+    print(a, b, c, d, e, f)
+
+func(1, *(2, 3), f=6, *(4, 5))
+func(*(1, 2), e=5, *(3, 4), f=6)
+func(1, **{'b': 2, 'c': 3}, d=4, **{'e': 5, 'f': 6})
+func(c=3, *(1, 2), **{'d': 4}, e=5, **{'f': 6})
diff --git a/ch04/arguments.keyword.py b/ch04/arguments.keyword.py
new file mode 100644
index 0000000..3bc4ea9
--- /dev/null
+++ b/ch04/arguments.keyword.py
@@ -0,0 +1,5 @@
+# arguments.keyword.py
+def func(a, b, c):
+    print(a, b, c)
+
+func(a=1, c=2, b=3)  # prints: 1 3 2
diff --git a/ch04/arguments.multiple.value.py b/ch04/arguments.multiple.value.py
new file mode 100644
index 0000000..b4cbe17
--- /dev/null
+++ b/ch04/arguments.multiple.value.py
@@ -0,0 +1,13 @@
+# arguments.multiple.value.py
+def func(a, b, c):
+    print(a, b, c)
+
+func(2, 3, a=1)
+
+"""
+$ python arguments.multiple.value.py
+Traceback (most recent call last):
+  File "arguments.multiple.value.py", line 5, in <module>
+    func(2, 3, a=1)
+TypeError: func() got multiple values for argument 'a'
+"""
diff --git a/ch04/arguments.positional.keyword.py b/ch04/arguments.positional.keyword.py
new file mode 100644
index 0000000..f6b591f
--- /dev/null
+++ b/ch04/arguments.positional.keyword.py
@@ -0,0 +1,15 @@
+# arguments.positional.keyword.py
+def func(a, b, c):
+    print(a, b, c)
+
+func(42, b=1, c=2)
+
+func(b=1, c=2, 42)  # positional argument after keyword arguments
+
+"""
+$ python arguments.positional.keyword.py
+  File "arguments.positional.keyword.py", line 7
+    func(b=1, c=2, 42)  # positional argument after keyword arguments
+                     ^
+SyntaxError: positional argument follows keyword argument
+"""
diff --git a/ch04/arguments.positional.py b/ch04/arguments.positional.py
new file mode 100644
index 0000000..b9ab8b4
--- /dev/null
+++ b/ch04/arguments.positional.py
@@ -0,0 +1,5 @@
+# arguments.positional.py
+def func(a, b, c):
+    print(a, b, c)
+
+func(1, 2, 3)  # prints: 1 2 3
diff --git a/ch04/arguments.unpack.dict.py b/ch04/arguments.unpack.dict.py
new file mode 100644
index 0000000..28f2fec
--- /dev/null
+++ b/ch04/arguments.unpack.dict.py
@@ -0,0 +1,6 @@
+# arguments.unpack.dict.py
+def func(a, b, c):
+    print(a, b, c)
+
+values = {'b': 1, 'c': 2, 'a': 42}
+func(**values)  # equivalent to func(b=1, c=2, a=42)
diff --git a/ch04/arguments.unpack.iterable.py b/ch04/arguments.unpack.iterable.py
new file mode 100644
index 0000000..31bf7ac
--- /dev/null
+++ b/ch04/arguments.unpack.iterable.py
@@ -0,0 +1,6 @@
+# arguments.unpack.iterable.py
+def func(a, b, c):
+    print(a, b, c)
+
+values = (1, 3, -7)
+func(*values)  # equivalent to: func(1, 3, -7)
diff --git a/ch04/data.science.example.py b/ch04/data.science.example.py
new file mode 100644
index 0000000..f2bcb69
--- /dev/null
+++ b/ch04/data.science.example.py
@@ -0,0 +1,22 @@
+# data.science.example.py
+def do_report(data_source):
+    # fetch and prepare data
+    data = fetch_data(data_source)
+    parsed_data = parse_data(data)
+    filtered_data = filter_data(parsed_data)
+    polished_data = polish_data(filtered_data)
+
+    # run algorithms on data
+    final_data = analyse(polished_data)
+
+    # create and return report
+    report = Report(final_data)
+    return report
+
+
+if __name__ == "__main__":
+
+    print(
+        "Please don't call the `do_report` function. "
+        "It's just and example."
+    )
diff --git a/ch04/docstrings.py b/ch04/docstrings.py
new file mode 100644
index 0000000..f335062
--- /dev/null
+++ b/ch04/docstrings.py
@@ -0,0 +1,24 @@
+# docstrings.py
+def square(n):
+    """Return the square of a number n. """
+    return n ** 2
+
+def get_username(userid):
+    """Return the username of a user given their id. """
+    return db.get(user_id=userid).username
+
+
+def connect(host, port, user, password):
+    """Connect to a database.
+
+    Connect to a PostgreSQL database directly, using the given
+    parameters.
+
+    :param host: The host IP.
+    :param port: The desired port.
+    :param user: The connection username.
+    :param password: The connection password.
+    :return: The connection object.
+    """
+    # body of the function here...
+    return connection
diff --git a/ch04/filter.lambda.py b/ch04/filter.lambda.py
new file mode 100644
index 0000000..b8bc386
--- /dev/null
+++ b/ch04/filter.lambda.py
@@ -0,0 +1,6 @@
+# filter.lambda.py
+def get_multiples_of_five(n):
+    return list(filter(lambda k: not k % 5, range(n)))
+
+
+print(get_multiples_of_five(50))
diff --git a/ch04/filter.regular.py b/ch04/filter.regular.py
new file mode 100644
index 0000000..1616437
--- /dev/null
+++ b/ch04/filter.regular.py
@@ -0,0 +1,10 @@
+# filter.regular.py
+def is_multiple_of_five(n):
+    return not n % 5
+
+
+def get_multiples_of_five(n):
+    return list(filter(is_multiple_of_five, range(n)))
+
+
+print(get_multiples_of_five(50))
diff --git a/ch04/func.attributes.py b/ch04/func.attributes.py
new file mode 100644
index 0000000..dfe85f2
--- /dev/null
+++ b/ch04/func.attributes.py
@@ -0,0 +1,16 @@
+# func.attributes.py
+def multiplication(a, b=1):
+    """Return a multiplied by b. """
+    return a * b
+
+
+if __name__ == "__main__":
+
+    special_attributes = [
+        "__doc__", "__name__", "__qualname__", "__module__",
+        "__defaults__", "__code__", "__globals__", "__dict__",
+        "__closure__", "__annotations__", "__kwdefaults__",
+    ]
+
+    for attribute in special_attributes:
+        print(attribute, '->', getattr(multiplication, attribute))
diff --git a/ch04/func_from.py b/ch04/func_from.py
new file mode 100644
index 0000000..c79b72a
--- /dev/null
+++ b/ch04/func_from.py
@@ -0,0 +1,5 @@
+# func_from.py
+from lib.funcdef import square, cube
+
+print(square(10))
+print(cube(10))
diff --git a/ch04/func_import.py b/ch04/func_import.py
new file mode 100644
index 0000000..a9a365c
--- /dev/null
+++ b/ch04/func_import.py
@@ -0,0 +1,6 @@
+# func_import.py
+import lib.funcdef
+
+
+print(lib.funcdef.square(10))
+print(lib.funcdef.cube(10))
diff --git a/ch04/imports.py b/ch04/imports.py
new file mode 100644
index 0000000..c69d663
--- /dev/null
+++ b/ch04/imports.py
@@ -0,0 +1,11 @@
+# imports.py
+from datetime import datetime, timezone  # two imports on the same line
+from unittest.mock import patch  # single import
+
+import pytest  # third party library
+
+from core.models import (  # multiline import
+    Exam,
+    Exercise,
+    Solution,
+)
diff --git a/ch04/key.points.argument.passing.py b/ch04/key.points.argument.passing.py
new file mode 100644
index 0000000..5a3ca43
--- /dev/null
+++ b/ch04/key.points.argument.passing.py
@@ -0,0 +1,6 @@
+# key.points.argument.passing.py
+x = 3
+def func(y):
+    print(y)
+
+func(x)  # prints: 3
diff --git a/ch04/key.points.assignment.py b/ch04/key.points.assignment.py
new file mode 100644
index 0000000..440f577
--- /dev/null
+++ b/ch04/key.points.assignment.py
@@ -0,0 +1,7 @@
+# key.points.assignment.py
+x = 3
+def func(x):
+    x = 7  # defining a local x, not changing the global one
+
+func(x)
+print(x)  # prints: 3
diff --git a/ch04/key.points.mutable.assignment.py b/ch04/key.points.mutable.assignment.py
new file mode 100644
index 0000000..458ca86
--- /dev/null
+++ b/ch04/key.points.mutable.assignment.py
@@ -0,0 +1,8 @@
+# key.points.mutable.assignment.py
+x = [1, 2, 3]
+def func(x):
+    x[1] = 42  # this changes the original `x` argument!
+    x = 'something else'  # this points x to a new string object
+
+func(x)
+print(x)  # still prints: [1, 42, 3]
diff --git a/ch04/key.points.mutable.py b/ch04/key.points.mutable.py
new file mode 100644
index 0000000..16ac26d
--- /dev/null
+++ b/ch04/key.points.mutable.py
@@ -0,0 +1,7 @@
+# key.points.mutable.py
+x = [1, 2, 3]
+def func(x):
+    x[1] = 42  # this affects the `x` argument!
+
+func(x)
+print(x)  # prints: [1, 42, 3]
diff --git a/ch04/lambda.explained.py b/ch04/lambda.explained.py
new file mode 100644
index 0000000..cbed5e5
--- /dev/null
+++ b/ch04/lambda.explained.py
@@ -0,0 +1,30 @@
+# lambda.explained.py
+"""
+myfunc = lambda [parameter_list]: expression
+
+def myfunc([parameter_list]):
+    return expression
+"""
+
+# example 1: adder
+def adder(a, b):
+    return a + b
+
+# is equivalent to:
+adder_lambda = lambda a, b: a + b
+
+# example 2: to uppercase
+def to_upper(s):
+    return s.upper()
+
+# is equivalent to:
+to_upper_lambda = lambda s: s.upper()
+
+
+if __name__ == "__main__":
+
+    print(adder(3, 4))
+    print(adder_lambda(3, 4))
+
+    print(to_upper("Hello"))
+    print(to_upper_lambda("Hello"))
diff --git a/ch04/lib/__init__.py b/ch04/lib/__init__.py
new file mode 100644
index 0000000..e69de29
diff --git a/ch04/lib/funcdef.py b/ch04/lib/funcdef.py
new file mode 100644
index 0000000..6130290
--- /dev/null
+++ b/ch04/lib/funcdef.py
@@ -0,0 +1,6 @@
+# lib/funcdef.py
+def square(n):
+    return n ** 2
+
+def cube(n):
+    return n ** 3
diff --git a/ch04/matrix.multiplication.func.py b/ch04/matrix.multiplication.func.py
new file mode 100644
index 0000000..6dca351
--- /dev/null
+++ b/ch04/matrix.multiplication.func.py
@@ -0,0 +1,12 @@
+# matrix.multiplication.func.py
+# this function could also be defined in another module
+def matrix_mul(a, b):
+    return [[sum(i * j for i, j in zip(r, c)) for c in zip(*b)]
+            for r in a]
+
+
+a = [[1, 2], [3, 4]]
+b = [[5, 1], [2, 1]]
+
+c = matrix_mul(a, b)
+print(c)
diff --git a/ch04/matrix.multiplication.nofunc.py b/ch04/matrix.multiplication.nofunc.py
new file mode 100644
index 0000000..f023bba
--- /dev/null
+++ b/ch04/matrix.multiplication.nofunc.py
@@ -0,0 +1,7 @@
+# matrix.multiplication.nofunc.py
+a = [[1, 2], [3, 4]]
+b = [[5, 1], [2, 1]]
+
+c = [[sum(i * j for i, j in zip(r, c)) for c in zip(*b)]
+     for r in a]
+print(c)
diff --git a/ch04/no.side.effects.py b/ch04/no.side.effects.py
new file mode 100644
index 0000000..eee7a02
--- /dev/null
+++ b/ch04/no.side.effects.py
@@ -0,0 +1,10 @@
+# no.side.effects.py
+# This file is not meant to be run
+>>> numbers = [4, 1, 7, 5]
+>>> sorted(numbers)  # won't sort the original `numbers` list
+[1, 4, 5, 7]
+>>> numbers  # let's verify
+[4, 1, 7, 5]  # good, untouched
+>>> numbers.sort()  # this will act on the list
+>>> numbers
+[1, 4, 5, 7]
diff --git a/ch04/parameters.all.pkwonly.py b/ch04/parameters.all.pkwonly.py
new file mode 100644
index 0000000..c87624e
--- /dev/null
+++ b/ch04/parameters.all.pkwonly.py
@@ -0,0 +1,8 @@
+# parameters.all.pkwonly.py
+def allparams(a, /, b,  c=42, *args, d=256, e, **kwargs):
+    print('a, b, c:', a, b, c)
+    print('d, e:', d, e)
+    print('args:', args)
+    print('kwargs:', kwargs)
+
+allparams(1, 2, 3, 4, 5, 6, e=7, f=9, g=10)
diff --git a/ch04/parameters.all.py b/ch04/parameters.all.py
new file mode 100644
index 0000000..cbad779
--- /dev/null
+++ b/ch04/parameters.all.py
@@ -0,0 +1,7 @@
+# parameters.all.py
+def func(a, b, c=7, *args, **kwargs):
+    print('a, b, c:', a, b, c)
+    print('args:', args)
+    print('kwargs:', kwargs)
+
+func(1, 2, 3, 5, 7, 9, A='a', B='b')
diff --git a/ch04/parameters.default.py b/ch04/parameters.default.py
new file mode 100644
index 0000000..1e9672e
--- /dev/null
+++ b/ch04/parameters.default.py
@@ -0,0 +1,8 @@
+# parameters.default.py
+def func(a, b=4, c=88):
+    print(a, b, c)
+
+func(1)              # prints: 1 4 88
+func(b=5, a=7, c=9)  # prints: 7 5 9
+func(42, c=9)        # prints: 42 4 9
+func(42, 43, 44)     # prints: 42, 43, 44
diff --git a/ch04/parameters.defaults.mutable.intermediate.call.py b/ch04/parameters.defaults.mutable.intermediate.call.py
new file mode 100644
index 0000000..ac216c2
--- /dev/null
+++ b/ch04/parameters.defaults.mutable.intermediate.call.py
@@ -0,0 +1,11 @@
+# parameters.defaults.mutable.intermediate.call.py
+def func(a=[], b={}):
+    print(a)
+    print(b)
+    print('#' * 12)
+    a.append(len(a))  # this will affect a's default value
+    b[len(a)] = len(a)  # and this will affect b's one
+
+func()
+func(a=[1, 2, 3], b={'B': 1})
+func()
diff --git a/ch04/parameters.defaults.mutable.no.trap.py b/ch04/parameters.defaults.mutable.no.trap.py
new file mode 100644
index 0000000..bf9c4de
--- /dev/null
+++ b/ch04/parameters.defaults.mutable.no.trap.py
@@ -0,0 +1,5 @@
+# parameters.defaults.mutable.no.trap.py
+def func(a=None):
+    if a is None:
+        a = []
+    # do whatever you want with `a` ...
diff --git a/ch04/parameters.defaults.mutable.py b/ch04/parameters.defaults.mutable.py
new file mode 100644
index 0000000..59ee6ce
--- /dev/null
+++ b/ch04/parameters.defaults.mutable.py
@@ -0,0 +1,11 @@
+# parameters.defaults.mutable.py
+def func(a=[], b={}):
+    print(a)
+    print(b)
+    print('#' * 12)
+    a.append(len(a))  # this will affect a's default value
+    b[len(a)] = len(a)  # and this will affect b's one
+
+func()
+func()
+func()
diff --git a/ch04/parameters.keyword.only.py b/ch04/parameters.keyword.only.py
new file mode 100644
index 0000000..b33b2b2
--- /dev/null
+++ b/ch04/parameters.keyword.only.py
@@ -0,0 +1,16 @@
+# parameters.keyword.only.py
+def kwo(*a, c):
+    print(a, c)
+
+kwo(1, 2, 3, c=7)  # prints: (1, 2, 3) 7
+kwo(c=4)           # prints: () 4
+# kwo(1, 2)  # breaks, invalid syntax, with the following error
+# TypeError: kwo() missing 1 required keyword-only argument: 'c'
+
+def kwo2(a, b=42, *, c):
+    print(a, b, c)
+
+kwo2(3, b=7, c=99)  # prints: 3 7 99
+kwo2(3, c=13)       # prints: 3 42 13
+# kwo2(3, 23)  # breaks, invalid syntax, with the following error
+# TypeError: kwo2() missing 1 required keyword-only argument: 'c'
diff --git a/ch04/parameters.positional.only.optional.py b/ch04/parameters.positional.only.optional.py
new file mode 100644
index 0000000..1c5ba36
--- /dev/null
+++ b/ch04/parameters.positional.only.optional.py
@@ -0,0 +1,6 @@
+# parameters.positional.only.optional.py
+def func(a, b=2, /):
+    print(a, b)
+
+func(4, 5)  # prints 4 5
+func(3)  # prints 3 2
diff --git a/ch04/parameters.positional.only.py b/ch04/parameters.positional.only.py
new file mode 100644
index 0000000..828ff40
--- /dev/null
+++ b/ch04/parameters.positional.only.py
@@ -0,0 +1,58 @@
+# parameters.positional.only.py
+def func(a, b, /, c):
+    print(a, b, c)
+
+func(1, 2, 3)  # prints: 1 2 3
+func(1, 2, c=3)  # prints 1 2 3
+# func(1, b=2, c=3)  # produces the following traceback:
+"""
+Traceback (most recent call last):
+  File "arguments.positional.only.py", line 7, in <module>
+    func(1, b=2, c=3)
+TypeError: func() got some positional-only arguments
+passed as keyword arguments: 'b'
+"""
+
+"""
+def func_name(positional_only_parameters, /,
+    positional_or_keyword_parameters, *,
+    keyword_only_parameters):
+
+
+Valid:
+
+def func_name(p1, p2, /, p_or_kw, *, kw):
+def func_name(p1, p2=None, /, p_or_kw=None, *, kw):
+def func_name(p1, p2=None, /, *, kw):
+def func_name(p1, p2=None, /):
+def func_name(p1, p2, /, p_or_kw):
+def func_name(p1, p2, /):
+
+
+Invalid:
+
+def func_name(p1, p2=None, /, p_or_kw, *, kw):
+def func_name(p1=None, p2, /, p_or_kw=None, *, kw):
+def func_name(p1=None, p2, /):
+
+
+Why it is useful:
+
+def divmod(a, b, /):
+    "Emulate the built in divmod() function"
+    return (a // b, a % b)
+
+
+len(obj='hello')  # The "obj" keyword argument impairs readability
+"""
+
+def func_name(name, /, **kwargs):
+    print(name)
+    print(kwargs)
+
+func_name('Positional-only name', name='Name in **kwargs')
+"""
+Prints:
+Positional-only name
+{'name': 'Name in **kwargs'}
+"""
diff --git a/ch04/parameters.variable.db.py b/ch04/parameters.variable.db.py
new file mode 100644
index 0000000..c8205e2
--- /dev/null
+++ b/ch04/parameters.variable.db.py
@@ -0,0 +1,15 @@
+# parameters.variable.db.py
+def connect(**options):
+    conn_params = {
+        'host': options.get('host', '127.0.0.1'),
+        'port': options.get('port', 5432),
+        'user': options.get('user', ''),
+        'pwd': options.get('pwd', ''),
+    }
+    print(conn_params)
+    # we then connect to the db (commented out)
+    # db.connect(**conn_params)
+
+connect()
+connect(host='127.0.0.42', port=5433)
+connect(port=5431, user='fab', pwd='gandalf')
diff --git a/ch04/parameters.variable.keyword.py b/ch04/parameters.variable.keyword.py
new file mode 100644
index 0000000..9240a1c
--- /dev/null
+++ b/ch04/parameters.variable.keyword.py
@@ -0,0 +1,7 @@
+# parameters.variable.keyword.py
+def func(**kwargs):
+    print(kwargs)
+
+func(a=1, b=42)  # prints {'a': 1, 'b': 42}
+func()  # prints {}
+func(a=1, b=46, c=99)  # prints {'a': 1, 'b': 46, 'c': 99}
diff --git a/ch04/parameters.variable.positional.py b/ch04/parameters.variable.positional.py
new file mode 100644
index 0000000..a52d4e5
--- /dev/null
+++ b/ch04/parameters.variable.positional.py
@@ -0,0 +1,12 @@
+# parameters.variable.positional.py
+def minimum(*n):
+    # print(type(n))  # n is a tuple
+    if n:  # explained after the code
+        mn = n[0]
+        for value in n[1:]:
+            if value < mn:
+                mn = value
+        print(mn)
+
+minimum(1, 3, -7, 9)  # n = (1, 3, -7, 9) - prints: -7
+minimum()             # n = () - prints: nothing
diff --git a/ch04/primes.py b/ch04/primes.py
new file mode 100644
index 0000000..27f1c00
--- /dev/null
+++ b/ch04/primes.py
@@ -0,0 +1,47 @@
+# primes.py
+from math import sqrt, ceil
+
+
+def get_primes(n):
+    """Calculate a list of primes up to n (included). """
+    primelist = []
+    for candidate in range(2, n + 1):
+        is_prime = True
+        root = ceil(sqrt(candidate))  # division limit
+        for prime in primelist:  # we try only the primes
+            if prime > root:  # no need to check any further
+                break
+            if candidate % prime == 0:
+                is_prime = False
+                break
+        if is_prime:
+            primelist.append(candidate)
+    return primelist
+
+
+if __name__ == "__main__":
+
+    def test():
+        primes = get_primes(10**3)
+        primes2 = [
+            2, 3, 5, 7, 11, 13, 17, 19, 23, 29, 31, 37, 41, 43,
+            47, 53, 59, 61, 67, 71, 73, 79, 83, 89, 97, 101, 103,
+            107, 109, 113, 127, 131, 137, 139, 149, 151, 157, 163,
+            167, 173, 179, 181, 191, 193, 197, 199, 211, 223, 227,
+            229, 233, 239, 241, 251, 257, 263, 269, 271, 277, 281,
+            283, 293, 307, 311, 313, 317, 331, 337, 347, 349, 353,
+            359, 367, 373, 379, 383, 389, 397, 401, 409, 419, 421,
+            431, 433, 439, 443, 449, 457, 461, 463, 467, 479, 487,
+            491, 499, 503, 509, 521, 523, 541, 547, 557, 563, 569,
+            571, 577, 587, 593, 599, 601, 607, 613, 617, 619, 631,
+            641, 643, 647, 653, 659, 661, 673, 677, 683, 691, 701,
+            709, 719, 727, 733, 739, 743, 751, 757, 761, 769, 773,
+            787, 797, 809, 811, 821, 823, 827, 829, 839, 853, 857,
+            859, 863, 877, 881, 883, 887, 907, 911, 919, 929, 937,
+            941, 947, 953, 967, 971, 977, 983, 991, 997
+        ]
+        return primes == primes2
+
+    print(test())
+
+    print(get_primes(100))
diff --git a/ch04/recursive.factorial.py b/ch04/recursive.factorial.py
new file mode 100644
index 0000000..a629b3d
--- /dev/null
+++ b/ch04/recursive.factorial.py
@@ -0,0 +1,8 @@
+# recursive.factorial.py
+def factorial(n):
+    if n in (0, 1):  # base case
+        return 1
+    return factorial(n - 1) * n  # recursive case
+
+
+print([factorial(n) for n in range(10)])
diff --git a/ch04/return.multiple.py b/ch04/return.multiple.py
new file mode 100644
index 0000000..00fac92
--- /dev/null
+++ b/ch04/return.multiple.py
@@ -0,0 +1,26 @@
+# return.multiple.py
+def moddiv(a, b):
+    return a // b, a % b
+
+
+def test(n=10**4):
+    from random import choice, random, randint
+    m = 10 ** 6
+    for x in range(n):
+        a = choice((randint(0, m), m * random()))
+        b = 0
+        while not b:
+            b = choice((randint(1, m), m * random()))
+
+        r = divmod(a, b)
+        r2 = moddiv(a, b)
+        if r != r2:
+            print('Difference: ', a, b, r, r2)
+
+
+if __name__ == "__main__":
+
+    test(10 ** 6)
+    print('Done')
+
+    print(moddiv(20, 7))  # prints (2, 6)
diff --git a/ch04/return.none.py b/ch04/return.none.py
new file mode 100644
index 0000000..9e246f8
--- /dev/null
+++ b/ch04/return.none.py
@@ -0,0 +1,7 @@
+# return.none.py
+def func():
+    pass
+
+func()  # the return of this call won't be collected. It's lost.
+a = func()  # the return of this one instead is collected into `a`
+print(a)  # prints: None
diff --git a/ch04/return.single.value.2.py b/ch04/return.single.value.2.py
new file mode 100644
index 0000000..90f9c00
--- /dev/null
+++ b/ch04/return.single.value.2.py
@@ -0,0 +1,12 @@
+# return.single.value.2.py
+from functools import reduce
+from operator import mul
+
+
+def factorial(n):
+    return reduce(mul, range(1, n + 1), 1)
+
+
+f5 = factorial(5)  # f5 = 120
+print(f5)
+print([factorial(k) for k in range(10)])
diff --git a/ch04/return.single.value.py b/ch04/return.single.value.py
new file mode 100644
index 0000000..dad0099
--- /dev/null
+++ b/ch04/return.single.value.py
@@ -0,0 +1,11 @@
+# return.single.value.py
+def factorial(n):
+    if n in (0, 1):
+        return 1
+    result = n
+    for k in range(2, n):
+        result *= k
+    return result
+
+f5 = factorial(5)  # f5 = 120
+print(f5)
diff --git a/ch04/scoping.level.1.py b/ch04/scoping.level.1.py
new file mode 100644
index 0000000..b40a57a
--- /dev/null
+++ b/ch04/scoping.level.1.py
@@ -0,0 +1,9 @@
+# scoping.level.1.py
+def my_function():
+    test = 1 # this is defined in the local scope of the function
+    print('my_function:', test)
+
+
+test = 0  # this is defined in the global scope
+my_function()
+print('global:', test)
diff --git a/ch04/scoping.level.2.global.py b/ch04/scoping.level.2.global.py
new file mode 100644
index 0000000..67349e1
--- /dev/null
+++ b/ch04/scoping.level.2.global.py
@@ -0,0 +1,16 @@
+# scoping.level.2.global.py
+def outer():
+    test = 1  # outer scope
+
+    def inner():
+        global test
+        test = 2  # global scope
+        print('inner:', test)
+
+    inner()
+    print('outer:', test)
+
+
+test = 0  # global scope
+outer()
+print('global:', test)
diff --git a/ch04/scoping.level.2.nonlocal.py b/ch04/scoping.level.2.nonlocal.py
new file mode 100644
index 0000000..1e1c574
--- /dev/null
+++ b/ch04/scoping.level.2.nonlocal.py
@@ -0,0 +1,16 @@
+# scoping.level.2.nonlocal.py
+def outer():
+    test = 1  # outer scope
+
+    def inner():
+        nonlocal test
+        test = 2  # nearest enclosing scope (which is 'outer')
+        print('inner:', test)
+
+    inner()
+    print('outer:', test)
+
+
+test = 0  # global scope
+outer()
+print('global:', test)
diff --git a/ch04/scoping.level.2.py b/ch04/scoping.level.2.py
new file mode 100644
index 0000000..ac2dece
--- /dev/null
+++ b/ch04/scoping.level.2.py
@@ -0,0 +1,15 @@
+# scoping.level.2.py
+def outer():
+    test = 1  # outer scope
+
+    def inner():
+        test = 2  # inner scope
+        print('inner:', test)
+
+    inner()
+    print('outer:', test)
+
+
+test = 0  # global scope
+outer()
+print('global:', test)
diff --git a/ch04/vat.function.py b/ch04/vat.function.py
new file mode 100644
index 0000000..954ffcc
--- /dev/null
+++ b/ch04/vat.function.py
@@ -0,0 +1,7 @@
+# vat.function.py
+def calculate_price_with_vat(price, vat):
+    return price * (100 + vat) / 100
+
+
+if __name__ == "__main__":
+    print(calculate_price_with_vat(100, 20))
diff --git a/ch04/vat.py b/ch04/vat.py
new file mode 100644
index 0000000..cb0a1d3
--- /dev/null
+++ b/ch04/vat.py
@@ -0,0 +1,12 @@
+# vat.py
+price = 100  # GBP, no VAT
+final_price1 = price * 1.2
+final_price2 = price + price / 5.0
+final_price3 = price * (100 + 20) / 100.0
+final_price4 = price + price * 0.2
+
+
+if __name__ == "__main__":
+    for var, value in sorted(vars().items()):
+        if 'price' in var:
+            print(var, value)