Python. Curiosities, code, notes to get ready to PCEP
Python: curiosities, code & notes
I might take the PCEP exam to demonstrate my knowledge in Python. For now, I have taken a CISCO course called Python Essential 1 and here are some notes, curiosities, and code that I found interesting if you are not a Pythonista.
Quick notes
- CPython is the default and reference implementation of the Python language, written in C. When you install Python on your system, you are actually installing CPython. 😱
- PEP (Python Enhancement Proposals) is a system for designing and specifying Python. PEPs are documents that describe and discuss proposed features and changes for Python.
- What are the four fundamental elements of a language? An alphabet, a lexicon, a syntax, and a semantics.
- Where do functions come from? From Python itself (built-in functions), from modules, or from your code.
0o123is an octal number (base 8). They start with0oand this example is equal to 83 in decimal (base 10).0x123is a hexadecimal number (base 16). They start with0x. 291 is the decimal result in this case (base 10)..4 = 0.4 | 4. = 4.0- The result produced by the division operator is always a float, even if the operands are integers and the result is an integer.
print(10 / 5) # 2.0 - The asterisk (_) sign, when applied to a string and a number (or a number and a string) becomes a replication operator:
"an-" _ 3 # an-an-an- - What is the default return value for a function that does not explicitly return any value? It returns the value
None. - What is the result of the following comparison?
2 == 2.. The answer is True. Python converts the integer to a float before comparing them. - Useful exceptions:
ValueError,TypeError,AttributeError,SyntaxError.
What happens when Python encounters an invocation like the one below?
function_name(argument)
- First, Python checks if the specified name is legal (it explores its internal data to find an existing function of that name; if this search fails, Python aborts the code).
- Second, Python checks if the function's requirements for the number of arguments allow it to invoke the function in this way (for example, if a specific function requires exactly two arguments, any invocation that provides only one argument will be considered erroneous and will abort the code execution).
- Third, Python leaves the code for a moment and jumps into the function to be invoked; therefore, it also takes the argument(s) and passes them to the function.
- Fourth, the function executes the code, causes the desired effect (if any), evaluates the desired result(s), and completes the task.
- Finally, Python returns to the code (to the place immediately after the invocation) and resumes its execution.
Sequence types and mutability
Lists = []
list1 = [3] * 3 # [3, 3, 3]
list2 = [3, 3, 3] # [3, 3, 3]
list3 = [3 for i in range(3)] # [3, 3, 3]
We have three different ways to generate a list in Python
- Lists are mutable (they can be modified).
- They are sequences (they can be indexed and sliced).
- They are heterogeneous (they can contain elements of different types).
- They are ordered (the order of elements is important).
- Lists can be nested (a list can contain other lists).
Tuples = ()
my_tuple = (1, 2, 3)
- Tuples are immutable.
- They are sequences.
- They are heterogeneous.
- They are ordered.
- Tuples can be nested.
Dictionaries = {}
dictionary = {"cat": "chat", "dog": "chien", "horse": "cheval"}
- Dictionaries are mutable.
- Dictionaries are not sequences.
- Dictionaries are heterogeneous.
- Dictionaries are not ordered.
- Dictionaries can be nested.
my_dict = {"key1": "value1", "key2": "value2"}
my_dict["key3"] = "value3"
for key in my_dict.keys():
print(key, "->", my_dict[key])
# key1 -> value1
# key2 -> value2
# key3 -> value3
dictionary = {"cat": "chat", "dog": "chien", "horse": "cheval"}
for key in sorted(dictionary.keys()):
print(key, "->", dictionary[key])
# horse -> cheval
# cat -> chat
# dog -> chien
del dictionary['dog']
Info
Remember that we already reviewed lists and tuples in the article Python: Lists and Tuples.
Questions and answers
- What is the output of the following code?
a = 1
b = 0
x = a or b
y = not(a and b)
print(x + y)
# 2
The expression a or b evaluates to 1 because in Python, the or operator returns the first truthy value it encounters. Since 1 is truthy, x = 1.
The expression a and b evaluates to 0 because b is falsy (0). The and operator returns the last value if both values are truthy, otherwise it returns the first falsy value encountered. Hence, a and b gives 0. not(0) evaluates to True, which in Python is equivalent to 1. Therefore, y = 1.
EXTRA!: Cool code
Print() arguments
print("Hello", "World", sep="***", end="!")
# Hello***World!
Sorting a list
my_list = [8, 10, 6, 2, 4] # list to sort
swapped = True # We need it to be true to enter the while loop.
while swapped:
swapped = False # no swaps so far
for i in range(len(my_list) - 1):
if my_list[i] > my_list[i + 1]:
swapped = True # swap occurred!
my_list[i], my_list[i + 1] = my_list[i + 1], my_list[i]
print(my_list)
# [2, 4, 6, 8, 10]
Protection against iterating through None values
for element in mylist or []:
do_stuff (element)
Explanation: If mylist is None, the loop will iterate through an empty list.
Lists store memory locations
list_1 = [1]
list_2 = list_1
list_1[0] = 2
print(list_2)
# [2]
🤔
You could say that:
- The name of an ordinary variable is the name of its content:
variable = 1 - The name of a list is the name of a memory location where the list is stored:
list_1 = [1], where list_1 is a memory location name.
list_1 = [1]
list_2 = list_1[:]
list_1[0] = 2
print(list_2)
# [1]
This not visible part of the code described as [:] can produce a completely new list. my_list[start:end] is a way to copy a list.
my_list = [10, 8, 6, 4, 2]
new_list = my_list[1:3]
print(new_list)
# [8, 6]
The new_list will contain end - start (3 - 1 = 2) elements ‒ those with indices equal to 1 and 2 (but not 3).
my_list = [10, 8, 6, 4, 2]
new_list = my_list[3:]
print(new_list)
# [4, 2]