This post is a record made while learning Chapter 2 “Variables and Simple Data Types” in Eric Matthes’s book, Python Crash Course.
String
Create a string
A string is a series of characters, and anything inside quotes (single or double quotes) is considered a string in Python:
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| "This is a string.", 'This is also a string.'
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| ('This is a string.', 'This is also a string.')
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This flexibility make it is easy to use quotes and apostrophes in a string:
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| 'I told my friend, "Python is my favorite language!"', "The language 'Python' is named after Monty Python, not the snake.", "One of Python's strengths is its diverse and supportive community."
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| ('I told my friend, "Python is my favorite language!"',
"The language 'Python' is named after Monty Python, not the snake.",
"One of Python's strengths is its diverse and supportive community.")
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Change case in a string: title(), upper(), and lower()
title() method changes each word to title case, namely the first letter of each word uppercase and other letters lowercase.
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| name = "aDA loVeLAce"
print(name.title())
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| print("Ada".title(), "ADA".title(), "ada".title())
print("Ada".title() == "ADA".title(), "ADA".title() == "ada".title())
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| Ada Ada Ada
True True
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Similarly, we can change a string to all uppercase by upper() method, and all lowercase by lower() method:
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| name = "aDA loVeLAce"
print(name.upper())
print(name.lower())
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| ADA LOVELACE
ada lovelace
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On the other hand, we should note that title(), upper(), and lower() methods won’t change the value that was originally stored in name:
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| name = "aDA loVeLAce"
name.title()
print(name)
name = "aDA loVeLAce"
name.upper()
print(name)
name = "aDA loVeLAce"
name.lower()
print(name)
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| aDA loVeLAce
aDA loVeLAce
aDA loVeLAce
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Use variables in a string (f-string)
To insert a variable’s value into a string, place the letter “f” immediately before the opening quotation mark, and put braces around the name or names of any variable to be inserted:
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| first_name = "ada"
last_name = "lovelace"
full_name = f"{first_name} {last_name}"
print(full_name)
print(f"Hello, {full_name.title()}!")
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| ada lovelace
Hello, Ada Lovelace!
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These strings are called f-strings. The “f” is for format, because Python formats the string by replacing the name of any variable in braces with its value.
F-strings were first introduced in Python 3.6. In an earlier-version Python, we need to use the format() method rather than f-strings:
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| full_name = "{} {}".format(first_name, last_name)
print(full_name)
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Add whitespace (tabs or newlines) to a string: \t and \n
In programming, whitespace refers to any nonprinting character, such as spaces, tabs, and end-of-line symbols.
To add a tab in a string, use the character combination \t; to add a newline, use the character combination \n:
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| print("Languages:\n\tPython\n\tC\n\tJavaScript")
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| Languages:
Python
C
JavaScript
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Strip whitespace from a string: rstrip(), lstrip(), and strip()
Strip the extra whitespace on the right side of a string by rstrip() method, left ones by lstrip(), and both side by strip():
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| favorite_language = ' python '
favorite_language.rstrip(), favorite_language.lstrip(), favorite_language.strip()
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| (' python', 'python ', 'python')
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However, it should be noted that the whitespace is only removed from the string temporarily. To make whitespace removed permanently, we should associate the stripped value with the variable name, like:
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| favorite_language = 'python '
favorite_language = favorite_language.rstrip()
favorite_language
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In the real world, these stripping functions are often used to clean up user input before it’s stored in a program.
Number
Integer
In Python, two multiplication symbols are used to represent exponents:
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| 3 ** 2, 3 ** 3, 10 ** 6
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Float
Python calls any number with a decimal point a float. This term is used in most programming languages, and it refers to the fact that a decimal point can appear at any position in a number.
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| 0.1 + 0.1, 0.2 + 0.2, 2 * 0.1, 2* 0.2
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But sometimes we’ll get an arbitrary number of decimal places:
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| (0.2 + 0.1), (3 * 0.1), (0.2 + 0.1 == 0.3), (3 * 0.1 == 0.3), (0.2 + 0.1) == (3 * 0.1)
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| (0.30000000000000004, 0.30000000000000004, False, False, True)
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This problem has to do with floating point representation, and happens in all languages and is usually of little concern. A remedy to it is using native Python decimal module:
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| from decimal import *
a, b, c = Decimal('0.1'), Decimal('0.2'), Decimal('0.3')
a+b == c
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Mix integer and float in an operation
Python defaults to a float in any operation that uses a float, even if the output is a whole number (integer):
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| 4/2, 1 + 2.0, 2 * 3.0, 3.0 ** 2
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Underscores in number
Group digits using underscores to make large numbers more readable:
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| universe_age = 14_000_000_000
universe_age1 = 14_000000000
print(universe_age, universe_age1)
print(universe_age == universe_age1)
universe_age2 = 1_4000000000.12_342
print(universe_age2)
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| 14000000000 14000000000
True
14000000000.12342
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As can be see, this feature works for both integer and float, but only available in Python 3.6 and later.
Constant
A constant is like a variable whose value stays the same throughout the life of a program. Python doesn’t have built-in constant types, but Python programmers commonly use all capital letters to indicate a variable should be treated as a constant and never be changed:
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| MAX_CONNECTIONS = 5000
print(MAX_CONNECTIONS)
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Make a long number readable
Method 1: by exponents
Method 2: by inserting underscores
Method 3 (preferable): by scientific notation
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| print(1.234e10)
print(1.234E10)
print(float("1.234e10"))
print(float("1.234E10"))
print(float('1.234e10'))
print(float('1.234E10'))
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| 12340000000.0
12340000000.0
12340000000.0
12340000000.0
12340000000.0
12340000000.0
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References