## AN INFORMAL INTRODUCTION TO PYTHON 

In the following examples, input and output are distinguished by the presence or absence of prompts ( »> and … ): to repeat the example, you must type everything after the prompt, when the prompt appears; lines that do not begin with a prompt are output from the interpreter. Note that a secondary prompt on a line by itself in an example means you must type a blank line; this is used to end a multi-line command. 

Many of the examples in this manual, even those entered at the interactive prompt, include comments. Comments in Python start with the hash character, # , and extend to the end of the physical line. A comment may appear at the start of a line or following whitespace or code, but not within a string literal. A hash character within a string literal is just a hash character. Since comments are to clarify code and are not interpreted by Python, they may be omitted when typing in examples. 

Some examples: 

# this is the first comment spam =1 # and this is the second comment # ... and now a third! text ="# This is not a comment because it's inside quotes." 

## 3.1 Using Python as a Calculator 

Let’s try some simple Python commands. Start the interpreter and wait for the primary prompt,$_{>>>}$ . (It shouldn’t take long.) 

## 3.1.1 Numbers 

The interpreter acts as a simple calculator: you can type an expression at it and it will write the value. Expression syntax is straightforward: the operators$_{+}$ ,$_{-}$ ,$_{*}$and$_{/}$work just like in most other languages (for example, Pascal or C); parentheses ($_{()}$ ) can be used for grouping. For example: 

>>> 2+2 4 >>> 50-5*6 20 >>> (50-5*6)/4 5.0 >>> 8/5 # division always returns a floating point number 1.6 

The integer numbers (e.g. 2 ,$_{4}$ ,$_{20}$ ) have type$_{int}$, the ones with a fractional part (e.g. 5.0 ,$_{1.6}$ ) have type float . We will see more about numeric types later in the tutorial. 

Division ($_{/}$ ) always returns a float. To do floor division and get an integer result (discarding any fractional result) you can use the$_{//}$ operator; to calculate the remainder you can use$_{%}$ : 

