Data-Driven Engineering

Python numbers can be stored as a boolean bool (0=False, 1=True), integer int, a floating point number float, or as a string str. A variable name cannot start with a number, has no spaces (use '_' instead), and does not contain a special symbols

    ' " , < > / ? | \ ( ) ! @ # $ % ^ & * ~ - +

Avoid using reserved Python keywords as variable names:

    and assert break class continue def del
    elif else except exec finally for from
    global if import in is lambda not or pass
    print raise return try while

Use the type() function to determine the object type. Run each cell in a Jupyter Notebook with shortcut Shift-Enter.

    bool

    int

Multiple numbers can be assigned with a single line.

    <class 'float'> <class 'float'>

Numbers can also be stored as a string. A string can be created with single 'string' or double quotes "string". Enclose a string in double quotes if a single quote is in the string such as "I'm learning Python".

    str

⚙ Operators

• + - * / addition, subtraction, multiplication, division
• % modulo (remainder after division)
• // floor division (discard the fraction, no rounding)
• ** exponential

    2.5 1 2 25

🧭 Comparison Operators

• > greater than, >= or equal to
• < less than, <= or equal to
• == equal to (notice the double equal sign, single assigns a value)
• != or <> not equal to

    True False False True

Chained comparisons 5>3>1 can be used as an alternative to 5>3 and 3>1. Use the \ character to continue onto the next line.

    True True

↔ Conditional Statement

Use an if statement to choose code flow based on a True or False result. Indentation (whitespace) is important in Python to indicate which lines belong to the if, elif (else if), or else conditions.

    x<3
    2.7

➰ For Loop

Two loop options are for and while. A for loop is used when the number of iterations is known. Use the range(n) function to iterate n times through a for loop.

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The range function can be used with 1-3 inputs:

• range(n) - iterate n times
• range(start,stop) - iterate between start and stop-1
• range(start,stop,step) - iterate between start and stop-1 with increment step

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➿ While Loop

A while loop continues until the condition is False. A while loop is used when the number of cycles is not known until the looping starts. The break and continue statements control the flow in a while loop.

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Simulate rolling a 6-sided die 🎲 by selecting a random number between 1 and 6. Continue until a 3 is rolled. The import random statement makes the randint() function available to generate the random integer number between 1 and 6.

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    1
    5
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    6
    2
    2
    1
    3

📝 Print Numbers

Display a number with the print() function.

    2.7

Formatted output controls the way the number is displayed. Display Number: 8.10 with f'Number: {3*x:8.2f}'. The format 8.2f means that there are 8 spaces total and 2 decimal places. The 3*x takes x=2.7 from the cell above and multiplies it by 3.

    'Number:     8.10'

Other common format specifiers include:

• d: signed integer
• e or E: floating point exponential format (e=lowercase, E=uppercase)
• f or F: floating point decimal format
• g or G: same as e/E if exponent is >=6 or <=-4, f otherwise
• s: string

The first number 8 indicates how many total spaces in the final string that represents the number. The .2 indicates how many decimal places are displayed. Leave off the 8 to include two decimals with the minimum number of spaces to represent the number (no blanks) with {3*e:.2f}. Strings with extra spaces can be left aligned < or right aligned >. The formatted output of x with 10 spaces is f'{x:>10}'.

    '       2.7'

Display 4 decimal places in exponential form with f'{x:.4e}'.

    '2.7000e+00'

💡 Convert Between Types

Use functions to convert between types:

• int() - convert to an integer (no range limits, no decimal)
• float() - convert to a floating point number (no range limits, includes decimal)
• str() - convert to a string (characters, not stored as a number)
• bin() or hex() - convert to binary or hexadecimal form (string)

Python automatically switches from 64-bit to long integer when an integer is greater than 9223372036854775807 (2**63 - 1). Python variables are mutable. Mutable means that the type can change and does not need to be declared before first use. Converting from a float to an int truncates the decimal from 2.7 (float) to 2 (int).

    2

Convert x=2.7 to int before converting to binary (base-2) with bin().

    '0b10'

Convert x=2.7 to int before converting to hexadecimal (base-16) with hex().

    '0x2'

Use round() to get the nearest integer 3 instead of truncating to 2.

    3

Adding two string numbers concatenates the two as '1' + '4.9' = '14.9'

    '34.9'

Convert the string to a float before adding to get 1 + 4.9 = 5.9. Use s=float(s) to change the class type of s to a float in later cells as well. To change the class type back to a string, use s=str(s). In this case, the i+float(s) changes s to a float only during the operation.

    5.9

🩹 Try and Except

The error engine stops the code and reports fatal errors. To customize how to handle errors:

The try and except is an easy way to prevent code from stopping but can mask errors, especially during early development phases when errors are needed to find sytax or logical problems with the code.

    division by zero
    x:  None

🖊 Input Function

A program can be interactive with the input() function to request a value from the user. The value is returned as a string and should be converted to a number with float() or int(), depending on the anticipated entry. Try using the input() function.

🧶 String Methods

Built-in string methods include:

• upper - convert to uppercase
• lower - convert to lowercase
• find - find index position of first occurance of character or string
• replace - replace string
• split - split sentence into a list of words

Use dir(n) to find other string methods. Replace 'Your Name' with your name.

    your name YOUR NAME

⏱ Time

Use the time package to get the number of seconds since 1 January 1970. This is a standard reference time that many time packages use.

    1656198771.4637268

⏳ The difference in seconds end-start tracks elapsed clock time.

😴 The sleep function pauses for a certain amount of time with time.sleep(1) for 1 second. The elapsed time is more than 1.0000 because the time.time() function takes a small amount of time to run.

    Elapsed time:  1.005829095840454

📅 Date

The datetime is another module to process dates and times. Change the date to a personally memorable date such as a birthdate.

🗓 Convert the string to a datetime object with either strptime or fromisoformat if the date is in the standard format YYYY-MM-DD HH:MM:SS.

    datetime.datetime(1997, 5, 11, 11, 5, 23)

⌚ Use the now() function to get the current datetime as tm = datetime.now(). Show the current datetime with:

• tm.ctime() - readable format (e.g. Sat Jun 25 17:12:52 2022)
• str(tm) - standard format (e.g. 2022-06-25 17:12:52.536004)

    Sat Jun 25 17:12:52 2022


    '2022-06-25 17:12:52.536004'

🕑 Elements of the datetime are unpacked with:

• tm.year - year (4 digits)
• tm.month - month (1-12)
• tm.day - day (1-31)
• tm.hour - hour (0-23)
• tm.minute - minute (0-59)
• tm.second - second (0-59)

🕓-🕒 Calculate elapsed time as a difference diff=tm-dt or convert to elapsed seconds with diff.total_seconds().

    9176 days, 6:07:29.536004
    792828449.536004

🔦 Functions

Two function definitions are:

• lambda - simple one-line functions
• def - code block with input(s) and output(s)

The lambda function is for coding simple one-line expressions. It is especially useful in combination with map(), filter(), and reduce() functions. The lambda function f(x) returns x**2+1. A for loop evaluates the function for values [0,1,2,3].

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Alternatively, use the def statement to define the f(x) function with the same result.

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Multiple inputs and outputs are possible with optional arguments with default (=1) values.

    2 10

❓Get Help

Use the help() function to access a description of the method or object. Jupyter Notebooks display help with Shift-Tab. Use websites such as Stack Overflow to find answers related error messages.

    Help on built-in function lower:

    lower() method of builtins.str instance
        Return a copy of the string converted to lowercase.

💻 Exercise 1A

Change the type of s to a floating point number with s = float(s). Verify that s is a floating point number with print(s) and type(s).

💻 Exercise 1B

Ask the user to input an integer with the input() function. Check that the input number is an integer. The try section of the code attempts to convert to a floating point number. The if statement checks if the float number is an integer value. If all conditions are successfully met, the integer is printed.

Use the sample code and test with inputs 1.x, 0, 3. Add a check and an error message if the integer value is outside the range (1-10). Use the code above and modify it to produce an error message if not an integer or outside the range 1-10.

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✅ Knowledge Check

1. What is the correct way to determine the type of an object in Python?

2. What will the output be for the following code snippet: print(6>2>1) ?