This chapter is from the book
This chapter is from the book
This chapter is from the book
Allowing Python Script Input
Sometimes you might need a script user to provide data into your script from the keyboard. To accomplish this task, Python provides the input function. The input function is a built-in function and has the following syntax:
variable=input(user prompt)
In Listing 4.24, the variable cups_consumed is assigned the value returned by the input function. The script user is prompted to provide this information. An input function argument designates the prompt provided to the user. The script user types an answer and presses the Enter key. This action causes the input function to assign the answer 3 as a value to the variable cups_consumed.
Listing 4.24 Variable Assignment via Script Input
>>> cups_consumed=input("How many cups did you drink? ")
How many cups did you drink? 3
>>> print("You drank", cups_consumed, "cups!")
You drank 3 cups!
>>>
For the user prompt, you can enclose the prompt’s string characters in either single or double quotes. The prompt is shown enclosed in double quotes in Listing 4.24’s input function.
The input function treats all input as strings. This is different from how Python handles other variable assignments. Remember that if cups_consumed=3 were in your Python script, it would be assigned the data type integer, int. When using the input function, as shown in Listing 4.25, the data type is set to string, str.
Listing 4.25 Data Type Assignments via Input
>>> cups_consumed=3
>>> type(cups_consumed)
<class 'int'>
>>> cups_consumed=input("How many cups did you drink? ")
How many cups did you drink? 3
>>> type(cups_consumed)
<class 'str'>
>>>
To convert variables (input from the keyboard) from strings, you can use the int function. The int function will convert a number from a string data type to an integer data type. You can use the float function to convert a number from a string to a floating-point data type. Listing 4.26 shows how to convert the variable cups_consumed to an integer data type.
Listing 4.26 Data Type Conversion via the int Function
>>> cups_consumed=input("How many cups did you drink? ")
How many cups did you drink? 3
>>> type(cups_consumed)
<class 'str'>
>>> cups_consumed=int(cups_consumed)
>>> type(cups_consumed)
<class 'int'>
>>>
You can get really tricky here and use a nested function. Nested functions are functions within functions. The general format is as follows:
variable=functionA(functionB())
Listing 4.27 uses this method to properly change the input data type from a string to an integer.
Listing 4.27 Using Nested Functions with input
>>> cups_consumed=int(input("How many cups did you drink? "))
How many cups did you drink? 3
>>> type(cups_consumed)
<class 'int'>
>>>
Using nested functions makes a Python script more concise. However, the trade-off is that the script is a little harder to read.
Try It Yourself: Explore Python Input and Output with Variables
You are now going to explore Python input and output using variables. In the following steps, you write a script to play with, instead of using the interactive Python shell:
- If you have not already done so, power up your Raspberry Pi and log in to the system.
- If you do not have the GUI started automatically at boot, start it now by typing startx and pressing Enter.
- Open the Terminal by double-clicking the Terminal icon.
- If you want to follow along with the book, you will need to create a directory to hold your Python scripts. At the command-line prompt, type mkdir py3prog and press Enter.
- At the command-line prompt, type nano py3prog/script0401.py and press Enter. The command puts you into the nano text editor and creates the file py3prog/script0401.py.
Type the following code into the nano editor window, pressing Enter at the end of each line:
# My first real Python script. # Written by <your name here> # ############ Define Variables ########### # amount=4 #Number of vessels. vessels='glasses' #Type of vessels used. liquid='water' #What is contained in the vessels. location='on the table' #Location of vessels. # ############ Output Variable Description ################# # print("This script has four variables pre-defined in it.") print() # print("The variables are as follows:") # print("name: amount", "data type:", type(amount), "value:", amount) # print("name: vessels", "data type:", type(vessels),"value:", vessels) # print("name: liquid", "data type:", type(liquid),"value:", liquid) # print("name: location", "data type:", type(location),"value:", location) print() # ############ Output Sentence Using Variables ############# # print("There are", amount, vessels, "full of", liquid, location,end='.\n') print() #- Write out the information you just typed in the text editor to the script by pressing Ctrl+O. The script filename will show along with the prompt filename to write. Press Enter to write out the contents to the script0401.py script.
- Exit the nano text editor by pressing Ctrl+X.
Type python3 py3prog/script0401.py and press Enter to run the script. If you encounter any errors, note them so you can fix them in the next step. You should see output like the output shown in Figure 4.3. The output is okay, but it’s a little sloppy. You can clean it up in the next step.
Figure 4.3 Output for the Python script script0401.py.
- At the command-line prompt, type nano py3prog/script0401.py and press Enter. The command puts you into the nano text editor, where you can modify the script0401.py script.
Go to the Output Variable Description portion of the script and add a separator to the end of each line. The lines of code to be changed; how they should look when you are done is shown here:
print("name: amount", "data type:", type (amount), "value:", amount, sep='\t') # print("name: vessels", "data type:", type (vessels), "value:", vessels, sep='\t') # print("name: liquid", "data type:", type (liquid), "value:", liquid, sep='\t') # print("name: location","data type:",type (location), "value:",location,sep='\t')- Write out the modified script by pressing Ctrl+O. Don’t press Enter yet! Change the filename to script0402.py and then press Enter. When nano asks Save file under DIFFERENT NAME ?, type Y and press Enter.
- Exit the nano text editor by pressing Ctrl+X.
Type python3 py3prog/script0402.py and press Enter to run the script. You should see output like the output shown in Figure 4.4. Much neater!
Figure 4.4 The script0402.py output, properly tabbed.
- To try adding some input into your script, at the command-line prompt, type nano py3prog/script0402.py and press Enter.
Go to the bottom of the script and add the Python statements shown here:
################## Get Input #################### # print() print("Now you may change the variables' values.") print() # amount=int(input("How many vessels are there? ")) print() # vessels = input("What type of vessels are being used? ") print() # liquid = input("What type of liquid is in the vessel? ") print() # location=input("Where are the vessels located? ") print() # ################# Display New Input to Output ########### # print("So you believe there are", amount, vessels, "of", liquid, location, end='. \n') print() # #################### End of Script ######################- Write out the modified script by pressing Ctrl+O. Don’t press Enter yet! Change the filename to script0403.py and then press Enter. When nano asks Save file under DIFFERENT NAME ?, type Y and press Enter.
- Exit the nano text editor by pressing Ctrl+X.
Type python3 py3prog/script0403.py and press Enter to run the script. Answer the prompts any way you want. (You are supposed to be having fun here!) Figure 4.5 shows what your output should look like.
Figure 4.5 The complete script0403.py output.
)
)
)
Run this script as many times as you want. Experiment with the various types of answers you enter and see what the results are. Also try making some minor modifications to the script and see what happens. Experimenting and playing with your Python script will enhance your learning.