Evaluation of IF statement in Python

Question

Please help me try to understand the evaluation of this script. It must be something simple I'm not understanding. I want to scan a text file and output lines with specific string value.

So for example my txt file will contain simple stuff:

beep
boop 
bop 
Hey 
beep
boop
bop

and the script is looking for the line with "Hey" and trying to output that line

file_path = "C:\\downloads\\test.txt"
i = 0
file_in = open(file_path,"r+") # read & write
for lines in file_in.readlines():
    i+=1
    #if lines.rfind("this entity will be skipped..."):
    if lines.find("Hey"):
        print(i, lines)
file_in.close()

For some reason it outputs every line except the one it found a match on. How is this possible?

Don't try that. Try reading the docs for the find method: "Return the lowest index in S where substring sub is found...Return -1 on failure." That means if it's at the beginning of the string, find returns 0. So you want if lines.find(...) >= 0. — larsks
– larsks, Commented May 12, 2020 at 23:56

Daniel Giger · Accepted Answer · 2020-05-13 00:03:32Z

5

It's probably more straightforward to do if "Hey" in lines: instead of if lines.find("Hey"):. If you really want to use find(), you could do this: if lines.find("Hey") != -1:

edited May 13, 2020 at 0:03

answered May 12, 2020 at 23:57

Daniel Giger

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Comments

Marat · Accepted Answer · 2020-05-13 00:57:38Z

4

While Daniel's answer is, of course, correct and should be accepted, I want to fuel my OCD and offer some improvements:

# use context managers to automatically release file handler
with open(file_path) as file_in:
    # file objects are iterable, will return lines 
    # reading entire file in memory can crash if file is too big
    # enumerate() is a more readable alternative to manual counters
    for i, line in enumerate(file_in):  # 'line' should be singular 
        if "Hey" in line:  # same as Daniel
             print(i, line)

edited May 13, 2020 at 0:57

answered May 13, 2020 at 0:04

Marat

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Comments

dham · Accepted Answer · 2020-05-13 00:07:22Z

1

.find(sub) returns an integer - the first index of sub if sub is found, and -1 if it is not.

When "Hey" is present, it is at the first index (0), so that is what .find() returns. Otherwise, "Hey" is not found, so .find() returns -1.

Since python treats all integers as True except 0, then the conditional evaluates to True when 0 is returned, i.e. when "Hey" is found.

Change your use of .find() to something which fulfills your if statement the way you want.

answered May 13, 2020 at 0:07

dham

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