How to fetch line numbers for all 'if' 'else' and 'elif' positions in a python file

Example: Suppose, we have a .py file containing the below code snippet. How do we read and extract the positions of if-elif-else

If fisrtconditon:#line 1
    If sub-condition:#line2
       print(line no 3)
elif secnd_condn:#line 4
     Xyz..#line5
     Xyz..#line6
elif third condition:line7
     ...line 8
else:#line 9
    Some content #line10***

Output:

[1,4,7,9]

Solution

A simple solution is to iterate over the lines of the file, using enumerate to help you get the line number:

with open("somefile.py") as f:
    for line_no, line in enumerate(f, start = 1):
        if line[:2] ==  "if" and (line[2].isspace() or line[2] == "("):
            print(line_no, "if")
        elif line[:4] == "elif" and (line[4].isspace() or line[4] == "("):
            print(line_no, "elif")
        elif line[:4] == "else" and (line[4].isspace() or line[4] == ":"):
            print(line_no, "else")

Assumptions: this program assumes that somefile.py has correct syntax. Moreover, if this if statement appears indented, for example, inside a function definition, it would not work. The question's specifications did not dictate this requirement.

This program

opens the file "somefile.py" for reading (the default mode of open);
iterates over the lines of the file, using enumerate to get an index; by default, indices will start from 0 but we specify the start parameter so that it starts from 1;
if the line starts with an if plus a (whitespace character or opening paren) or an elif plus a (whitespace character or opening paren) or an else plus a (colon : or a whitespace character), then we print the line number as well as the corresponding if, elif, or else.
as we exit the with block, Python closes the file for us.

Example

If somefile.py is

if condition:
    if condition:
        something
elif condition:
    something
    something
elif condition:
    something
else:
    something

then the above program outputs

1 if
4 elif
7 elif
9 else