Introduction to Python

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The Command Prompt: Executing Python Programs

verifying the PATH environment variable for Python

To execute scripts from the command line, we must ensure that the OS can find Python.

Please begin by opening a new Terminal or Command Prompt window. At the prompt, type python -V (make sure it is a capital V). (Please note that your prompt may look different than mine.)

Python can be found: python version is displayed

C:\Users\david> python -V
Python 3.11.5

Python can be found, but at the wrong version:

david@192 ~ % python -V
Python 2.7.16

Python can't be found:

david@192 ~ %  python -V
'python' is not a recognized...   or   'python': command not found...

If your path is not set correctly to a 3.x version of Python, you can find instructions on setting it in the supplementary documents for this class. You'll need to set the PATH to point to Python to continue with the remaining steps in this lesson. You may also contact your course manager for assistance.

executing a python script from the command line

Here we ask Python to run a script directly (without our IDE's help).

If you are in the same directory as the script, you can execute a program by running Python and telling Python the name of the script:

On Windows:

C:\Users\david\Desktop\python_data\02> python 2.1.py

On Mac:

Davids-MBP-3:02 david% python 2.1.py

Please note: if your prompt looks like this: >>>, you have entered the python interactive prompt. Type quit() and hit [Enter] to leave it. If there are any issues finding Python, please contact your course manager for assistance.

the STDIN, STDOUT and STDERR data streams

Your output goes to the screen, but in truth, it's going to "standard out".

• when we use print(), we expect the string to be printed to the screen
• the truth is that any output goes to something called a data stream
• a data stream is simply an 'output pipe' from your program
• once it leaves your program, the data is handled by the operating system
• the standard output data stream is called "standard out", abbreviated as STDOUT
• a similar output data stream, "standard error" or STDERR, is used when your program is delivering error output (an example is the string output of exit(), for example exit('error: missing argument'))
• a third data stream, "standard in" or STDIN, can be fed into our programs

redirecting the STDOUT data stream to a file

STDOUT can be redirected to other places besides the screen.

hello.py: print a greeting

print('hello, world!')

redirecting STDOUT to a file at the command line (Windows or Mac):

mycomputer% python hello.py                # default:  to the screen
hello, world!

mycomputer% python hello.py > newfile.txt  # redirect to a file (not the screen)
                                           # (we see no output)

mycomputer% cat newfile.txt       # Mac:  cat spits out a file's contents
hello, world!

C:\> type newfile.txt             # Windows: type spits out a file's contents
hello, world!

• STDOUT is the "place" or "stream" that data goes to when we print it
• the > symbol can be used to redirect this stream to a file

redirecting the STDOUT data stream to STDIN of another program

The 'pipe' character can connect two programs; the output of a will be redirected as the input of b

Mac: direct output to the wc command (count lines, words and characters)

mycomputer% python hello.py | wc

       1       2      14                   # the output of wc

Windows: direct output to find command (count lines):

mycomputer% python hello.py | find /c /v ""

1

• hello.py prints to STDOUT (usually the screen)
• the pipe (|) connects this output to the STDIN of wc or find
• wc is a Unix utility that counts lines, words and characters in text
• find /c /v "" does similar work in Windows, counting lines

reading and redirecting the STDIN data stream

STDIN is the 'input pipe' to our program (usually the keyboard, but can be redirected to read from a file or other program).

import sys

for line in sys.stdin.readlines():
    print(line)

filetext = sys.stdin.read()          # alternative to above

A program like the above could be called this way, directing a file into our program's STDIN:

mycomputer% python readfile.py < file_to_be_read.txt

We can of course also direct the output of a program into our program's STDIN through use of a pipe:

mycomputer% ls -l | python readfile.py