How to print commands in Python?

Question

I'm not in the programming area but I recently got interested in Python. I was writing some functions but for debugging I need to see what commands are running. For instance:

def foo():
    for i in xrange(0,5):
        a = 1 + i

Is it possible to make the interpreter output

>>> for i in xrange(0,5)
>>> a = 1 + 0
>>> a = 1 + 1
>>> a = 1 + 2
>>> a = 1 + 3
>>> a = 1 + 4

For

>>> foo()

Or at least write to a file what is happening? I did some scripting in the past and I remember that this was possible in DOS, using @ECHO ON or something. I did some reading and I feel like it's related to stdin and stdout in Python so I tried

import sys
def foo():
    for i in xrange(0,5):
        a = 1 + i
        sys.stdin.flush()
        sys.stdout.flush()

But I get nothing... I also tried

import sys
# foo()
sys.stdin.read()
sys.stdout.read()

and https://stackoverflow.com/a/3289051/2032568, but it just hangs. Sorry if this is not the right place for beginners. I couldn't find anything that answers my question.

Fredrik Pihl · Accepted Answer · 2013-04-14 20:12:57Z

Have a look at the trace-module

python -m trace --count -C . somefile.py

output is placed in currebt directory:

$ cat somefile.trace
    1: def foo():
    6:     for i in xrange(5):
    5:         a = 1 + i

    1: foo()

-c, --count Produce a set of annotated listing files upon program completion that shows how many times each statement was executed

If using the -t option you get this:

$ python -m trace --count -t tr.py 
 --- modulename: tr, funcname: <module>
tr.py(1): def foo():
tr.py(5): foo()
 --- modulename: tr, funcname: foo
tr.py(2):     for i in xrange(5):
tr.py(3):         a = 1 + i
tr.py(2):     for i in xrange(5):
tr.py(3):         a = 1 + i
tr.py(2):     for i in xrange(5):
tr.py(3):         a = 1 + i
tr.py(2):     for i in xrange(5):
tr.py(3):         a = 1 + i
tr.py(2):     for i in xrange(5):
tr.py(3):         a = 1 + i
tr.py(2):     for i in xrange(5):

Community · Accepted Answer · 2017-05-23 12:28:07Z

To make the interpreter print out expression values during runtime you can use the print statement. Also take a note of Python's string formatting facilities.

Example:

for i in xrange(0,5):
    a = 1 + i
    # print the value of a:
    print "the current value of variable 'a':", a

There is no need to flush stdout explicitly, unless you want to enforce to print out lines without a terminating newline:

import sys
import time
for i in xrange(0,5):
    a = 1 + i
    # print the value of a:
    # the trailing comma prevents 'print' from adding a newline
    print "\rthe current value of variable 'a':", a, 
    sys.stdout.flush()
    # short pause for purposes of demonstration
    time.sleep(1)
# finally print a newline
print

To print each statement before it's executed have a look at the trace module.

Example:

y = 0
for xi in range(3):
    y += xi
print y

The output:

$ python -m trace -t tt.py
 --- modulename: tt, funcname: <module>
tt.py(2): y = 0
tt.py(3): for xi in range(3):
tt.py(4):     y += xi
tt.py(3): for xi in range(3):
tt.py(4):     y += xi
tt.py(3): for xi in range(3):
tt.py(4):     y += xi
tt.py(3): for xi in range(3):
tt.py(5): print y
3
 --- modulename: trace, funcname: _unsettrace
trace.py(80):         sys.settrace(None)

What you are looking for in the first place, might also be a debugger, e.g. pdb. You get an interactive session, where you can step through the code, and have a look at the data interactively.

Example:

$ python -m pdb tt.py
> /home/moooeeeep/tt.py(2)<module>()
-> y = 0
(Pdb) n
> /home/moooeeeep/tt.py(3)<module>()
-> for xi in range(3):
(Pdb) n
> /home/moooeeeep/tt.py(4)<module>()
-> y += xi
(Pdb) n
> /home/moooeeeep/tt.py(3)<module>()
-> for xi in range(3):
(Pdb) n
> /home/moooeeeep/tt.py(4)<module>()
-> y += xi
(Pdb) print y, xi
0 1
(Pdb) 
...

Most Python IDEs (e.g., PyDev) have nicely integrated debugging functionality. So my suggestion: go and get a debugger.

Ashwini Chaudhary · Accepted Answer · 2013-04-14 20:04:46Z

1

You mean something like this?

def foo():
         for i in xrange(0,5):
                 a = 1 + i
                 print "a = 1 + {0}".format(i)

>>> foo()
a = 1 + 0
a = 1 + 1
a = 1 + 2
a = 1 + 3
a = 1 + 4

answered Apr 14, 2013 at 20:04

Ashwini Chaudhary

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1 Comment

Warren P Over a year ago

Do you think the OP really wants to write explicit print statements?

Santiago · Accepted Answer · 2013-04-14 20:02:55Z

0

def foo():
    for i in xrange(0,5):
        a = 1 + i
        print a

i think this its what you are looking for, i hope this be usefull for you :)

edit:

i think i understood what you want:

def foo():
    for i in xrange(0,5):
        print "a = 1 + i"

answered Apr 14, 2013 at 20:02

Santiago

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1 Comment

user2280382 Over a year ago

Sure, but if I have, say: a = (x ** y) + (2 * b), I'd like to get a = (-1 ** 2) + (2 * 2), for x=-1, y=2, b=2. Not only the result, which would be 4.

Pythonidae · Accepted Answer · 2013-04-14 20:18:08Z

0

I understand what you what to achieve here and I checked that link: it hangs for me too, and only repeats inputted text in the Terminal. Unfortunately I don't know how to print out the script as you've asked: for debugging, I'd advise just using simple print commands to work out which section is being executed.

def foo():
    for i in xrange(0,5):
        a = 1 + i, print "line is being executed where i = %i " % i

Then just read the printed text output to see what the program is doing. Hope this helps.

answered Apr 14, 2013 at 20:18

Pythonidae

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