## Key Concepts

Review core concepts you need to learn to master this subject

### Comments

```# Comment on a single line user = "JDoe" # Comment after code```

A comment is a piece of text within a program that is not executed. It can be used to provide additional information to aid in understanding the code.

The `#` character is used to start a comment and it continues until the end of the line.

### Arithmetic Operations

```# Comment on a single line user = "JDoe" # Comment after code```

Python supports different types of arithmetic operations that can be performed on literal numbers, variables, or some combination. The primary arithmetic operators are:

• `+` for addition
• `-` for subtraction
• `*` for multiplication
• `/` for division
• `%` for modulus (returns the remainder)
• `**` for exponentiation

### Plus-Equals Operator `+=`

```# Comment on a single line user = "JDoe" # Comment after code```

The plus-equals operator `+=` provides a convenient way to add a value to an existing variable and assign the new value back to the same variable. In the case where the variable and the value are strings, this operator performs string concatenation instead of addition.

The operation is performed in-place, meaning that any other variable which points to the variable being updated will also be updated.

### Variables

```# Comment on a single line user = "JDoe" # Comment after code```

A variable is used to store data that will be used by the program. This data can be a number, a string, a Boolean, a list or some other data type. Every variable has a name which can consist of letters, numbers, and the underscore character `_`.

The equal sign `=` is used to assign a value to a variable. After the initial assignment is made, the value of a variable can be updated to new values as needed.

### Modulo Operator `%`

```# Comment on a single line user = "JDoe" # Comment after code```

A modulo calculation returns the remainder of a division between the first and second number. For example:

• The result of the expression `4 % 2` would result in the value 0, because 4 is evenly divisible by 2 leaving no remainder.
• The result of the expression `7 % 3` would return 1, because 7 is not evenly divisible by 3, leaving a remainder of 1.

### Integers

```# Comment on a single line user = "JDoe" # Comment after code```

An integer is a number that can be written without a fractional part (no decimal). An integer can be a positive number, a negative number or the number 0 so long as there is no decimal portion.

The number `0` represents an integer value but the same number written as `0.0` would represent a floating point number.

### String Concatenation

```# Comment on a single line user = "JDoe" # Comment after code```

Python supports the joining (concatenation) of strings together using the `+` operator. The `+` operator is also used for mathematical addition operations. If the parameters passed to the `+` operator are strings, then concatenation will be performed. If the parameter passed to `+` have different types, then Python will report an error condition. Multiple variables or literal strings can be joined together using the `+` operator.

### Errors

```# Comment on a single line user = "JDoe" # Comment after code```

The Python interpreter will report errors present in your code. For most error cases, the interpreter will display the line of code where the error was detected and place a caret character `^` under the portion of the code where the error was detected.

### ZeroDivisionError

```# Comment on a single line user = "JDoe" # Comment after code```

A ZeroDivisionError is reported by the Python interpreter when it detects a division operation is being performed and the denominator (bottom number) is 0. In mathematics, dividing a number by zero has no defined value, so Python treats this as an error condition and will report a ZeroDivisionError and display the line of code where the division occurred. This can also happen if a variable is used as the denominator and its value has been set to or changed to 0.

### Strings

```# Comment on a single line user = "JDoe" # Comment after code```

A string is a sequence of characters (letters, numbers, whitespace or punctuation) enclosed by quotation marks. It can be enclosed using either the double quotation mark `"` or the single quotation mark `'`.

If a string has to be broken into multiple lines, the backslash character `\` can be used to indicate that the string continues on the next line.

### SyntaxError

```# Comment on a single line user = "JDoe" # Comment after code```

A SyntaxError is reported by the Python interpreter when some portion of the code is incorrect. This can include misspelled keywords, missing or too many brackets or parenthesis, incorrect operators, missing or too many quotation marks, or other conditions.

### NameError

```# Comment on a single line user = "JDoe" # Comment after code```

A NameError is reported by the Python interpreter when it detects a variable that is unknown. This can occur when a variable is used before it has been assigned a value or if a variable name is spelled differently than the point at which it was defined. The Python interpreter will display the line of code where the NameError was detected and indicate which name it found that was not defined.

### Floating Point Numbers

```# Comment on a single line user = "JDoe" # Comment after code```

Python variables can be assigned different types of data. One supported data type is the floating point number. A floating point number is a value that contains a decimal portion. It can be used to represent numbers that have fractional quantities. For example, `a = 3/5` can not be represented as an integer, so the variable `a` is assigned a floating point value of `0.6`.

### `print()` Function

```# Comment on a single line user = "JDoe" # Comment after code```

The `print()` function is used to output text, numbers, or other printable information to the console.

It takes one or more arguments and will output each of the arguments to the console separated by a space. If no arguments are provided, the `print()` function will output a blank line.

Learn Python: Syntax
Lesson 1 of 1
1. 1
Python is a programming language. Like other languages, it gives us a way to communicate ideas. In the case of a programming language, these ideas are “commands” that people use to communicate with…
2. 2
Ironically, the first thing we’re going to do is show how to tell a computer to ignore a part of a program. Text written in a program but not run by the computer is called a comment. Python inter…
3. 3
Now what we’re going to do is teach our computer to communicate. The gift of speech is valuable: a computer can answer many questions we have about “how” or “why” or “what” it is doing. In Python, …
4. 4
Computer programmers refer to blocks of text as strings. In our last exercise, we created the string “Hello world!”. In Python a string is either surrounded by double quotes (“Hello world”) or si…
5. 5
Programming languages offer a method of storing data for reuse. If there is a greeting we want to present, a date we need to reuse, or a user ID we need to remember we can create a variable whic…
6. 6
Humans are prone to making mistakes. Humans are also typically in charge of creating computer programs. To compensate, programming languages attempt to understand and explain mistakes made in their…
7. 7
Computers can understand much more than just strings of text. Python has a few numeric data types. It has multiple ways of storing numbers. Which one you use depends on your intended purpose for …
8. 8
Computers absolutely excel at performing calculations. The “compute” in their name comes from their historical association with providing answers to mathematical questions. Python performs addition…
9. 9
Variables that are assigned numeric values can be treated the same as the numbers themselves. Two variables can be added together, divided by 2, and multiplied by a third variable without Python di…
10. 10
Python can also perform exponentiation. In written math, you might see an exponent as a superscript number, but typing superscript numbers isn’t always easy on modern keyboards. Since this operatio…
11. 11
Python offers a companion to the division operator called the modulo operator. The modulo operator is indicated by % and gives the remainder of a division calculation. If the number is divisible, t…
12. 12
The + operator doesn’t just add two numbers, it can also “add” two strings! The process of combining two strings is called string concatenation. Performing string concatenation creates a brand ne…
13. 13
Python offers a shorthand for updating variables. When you have a number saved in a variable and want to add to the current value of the variable, you can use the += (plus-equals) operator. # Firs…
14. 14
Python strings are very flexible, but if we try to create a string that occupies multiple lines we find ourselves face-to-face with a SyntaxError. Python offers a solution: multi-line strings. By…
15. 15
In this lesson, we accomplished a lot of things! We instructed our computers to print messages, we stored these messages as variables, and we learned to update those messages depending on the part …

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