How to parse the starting and ending of a loop statements in python

Question

My objective is to find the line numbers of the start and the end of a loop statement in python.

Example scenario

#A.py
Line1: a=0                  
Line2: while a<5:           
Line3:    print a          
Line4:    a=a+1 

Desired output:
Start of a loop Line2 
End of a loop   Line4

Current parser code

#parser.py
with open(a) as f:
    tree = ast.parse(f.read())
taskline=[]
for node in ast.walk(tree):
    if isinstance(node, (ast.For)) or isinstance(node,(ast.While)):                        
        print node.lineno-1  <-- This give line number on for the start of a loop

I wanted to achieve the above output. I use AST to parse a given file and determine the occurrence of loops. With AST parsing i am able to find line number for the start of the loop but the line number for ending of the loop is yet to be determined. Is there any way i could parse an entire loop statement and determine its starting and ending line number ?

torek · Accepted Answer · 2013-07-30 11:13:02Z

A While node has its statements in its node.body list. The last line of the while loop is the last element of the list. I don't know why you are subtracting one (unless your file a has a comment that you want to pretend does not exist):

$ cat a.py
a = 0 
while a < 5:
    print a
    a += 1
for i in (1, 2, 3): 
    pass
$ cat ast_ex.py
import ast

with open('a.py') as f:
    tree = ast.parse(f.read())

for node in ast.walk(tree):
    if isinstance(node, (ast.For, ast.While)):
        print 'node:', node, 'at line:', node.lineno
        print 'body of loop ends at:', node.body[-1].lineno
$ python ast_ex.py 
node: <_ast.While object at 0x8017a8e50> at line: 2
body of loop ends at: 4
node: <_ast.For object at 0x8017ac0d0> at line: 5
body of loop ends at: 6

The first line in the loop is in body[0] (which may be the same as body[-1] if there is only one statement in the loop).

Anand · Accepted Answer · 2013-07-30 10:46:15Z

0

It could be complecated but you can try following algorithm.

1. Count the number of white spaces before while. say it ident(you can use something like this len(a) - len(a.lstrip()) )
2. countinue reading the next line and counting the white spaces before the line say currIdent.
3. when ever currIdent = ident, then end of loop is line before it.

answered Jul 30, 2013 at 10:46

Anand

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Comments

Daniel Waltrip · Accepted Answer · 2013-07-30 11:37:30Z

0

I am not super familiar with the ast module, but the code below worked for me on a few test examples. It returns a list of 2-tuples, one for each loop in the file, where each tuple looks like (start_line, end_line).

def get_loop_boundaries(fname):
    boundaries = []

    with open(fname) as f:
        tree = ast.parse(f.read())

    for node in ast.walk(tree):
        if isinstance(node, (ast.For)) or isinstance(node,(ast.While)):
            loop_start = node.lineno

            # body attribute is a list of nodes, one for each line in the loop
            # the lineno of the last node will give us the ending line
            loop_end = node.body[-1].lineno

            # add 2-tuple of starting and ending lines for current loop to list
            boundaries.append((loop_start, loop_end))
    # return a list of starting and ending lines for all loops in fname file
    return boundaries

I just realized the main logic of the function can be written much more concisely as a list comprehension:

return [(node.lineno, node.body[-1].lineno) for node in ast.walk(tree) if isinstance(node, (ast.For, ast.While))]

edited Jul 30, 2013 at 11:37

answered Jul 30, 2013 at 11:08

Daniel Waltrip

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1 Comment

Daniel Waltrip Over a year ago

torek's answer clarified some things regarding the ast module for me, so I improved my function.

Nachiket Deo · Accepted Answer · 2021-06-05 05:48:01Z

0

Torek's answer is really good and I myself tried to use it in my program but there's another way we can do it. 'ast' class provides a functionality named 'end_lineno' just like lineno. This can be used to find lineno of end of loop. Please refer to docs

import ast
with open('a.py') as f:
     tree = ast.parse(f.read())

for node in ast.walk(tree):
    if isinstance(node, (ast.For, ast.While)):
       print 'node:', node, 'at line:', node.lineno
       print 'body of loop ends at:', node.end_lineno

answered Jun 5, 2021 at 5:48

Nachiket Deo

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