python - if conditions in string is true

Question

python beginner here. My file contains lines that consists of conditions that are formatted just like a python if statement (without the if in the beginning and colon in the end). Example:

temperature < 40 and weekday == "Thursday" and (country != "Norway" or country != "USA")
(temperature != 30 or temperature != 35) and weekday == "Friday" and country == "Canada"

I want to write code that reads the lines as if they were if statements and prints True or False depending on if the conditions were met. I am thinking something along the lines of:

temperature = 35
country = "Canada"
weekday = "Friday"
file = open('output.txt', r)
lines = file.readlines()
for line in lines:
    if line:
        # if conditions in string are met, print true
        print(True)
    else:
        # else print False
        print(False)

which should when run with above file lines should output

False
True

EDIT:

Would it be possible to reliably and consistently parse the file lines and process it, like how I'm assuming a python compiler would read an actual if statement?

`(country != "Norway" or country != "USA")` is always true. Delete this in 1 line: `and (country != "Norway" or country != "USA")`, and there are similar true condition in 2 line. — George, Aug 10 '22 at 11:51
Simple but dangerous (because it executes arbitrary code from the file) is the built-in "eval" function. — Michael Butscher, Aug 10 '22 at 11:51
Don't use `eval` on entrusted, un-sanitised data. If you do, an error or malicious code in the data, could attack you. If you created the file, then consider importing it, else find a way to sanitise it. — ctrl-alt-delor, Aug 10 '22 at 12:01
@George if you use `eval` to eval date, if will run the data as code. If the data is entrusted input. Then you are running entrusted code. The supplier of this data, can get you computer to do whatever they want. — ctrl-alt-delor, Aug 10 '22 at 12:15

score 2 · Answer 1 · answered Aug 10 '22 at 11:55

2

Alright so you've got a few basic issues, mostly forgetting to use "" for strings such as in country = Canada and weekday = Friday. Other than that, you can use the eval() method, however, it is considered bad practice and you should try to avoid it.

temperature = 35
country = "Canada"
weekday = "Friday"
file = open('output.txt', "r")
lines = file.readlines()
for line in lines:
    if eval(line):
        # if conditions in string are met, print true
        print(True)
    else:
        # else print False
        print(False)

Note: Forgot to mention, you need "" for the read specifier in open().

answered Aug 10 '22 at 11:55

Marco Kurepa

325
1
9

I should also say I just fixed the code enough to make it work as intended, I'll edit in some good practices if you'd like. – Marco Kurepa Aug 10 '22 at 11:57
Yeah my bad I also fixed them – finjasnappy Aug 10 '22 at 11:58
3

why is eval bad practice? – finjasnappy Aug 10 '22 at 11:59
It is good answer only if you don't need to print `True` or `False` – George Aug 10 '22 at 12:00
2

@finjasnappy https://stackoverflow.com/questions/1832940/why-is-using-eval-a-bad-practice – Cow Aug 10 '22 at 12:02
2

@finjasnappy See [this post](https://stackoverflow.com/q/1832940/2476977) regarding eval. – Ben Grossmann Aug 10 '22 at 12:03
@MarcoKurepa I'm curious about your recommended good practices here – Ben Grossmann Aug 10 '22 at 12:09
`# if condition is True, then print True else if condition is False, then print False` – ctrl-alt-delor Aug 10 '22 at 12:21

score -4 · Answer 2 · answered Aug 10 '22 at 11:54

-4

There are two concepts, selection (if statements), and conditions / boolean logic <, or, and, etc. You are trying to use the 2nd. You are correct you don't need the 1st.

When we program we don't write big programs that don't work, the fix them. We start with small programs that work, and then make them bigger. So let me start with a simple example.

temperature < 40

How do we print True if this is true, and False if it is false? The answer may surprise you.

print(temperature < 40)

answered Aug 10 '22 at 11:54

ctrl-alt-delor

7,506
5
40
52

It is good answer only if you need to print `True` or `False` – George Aug 10 '22 at 11:59
I believe this answers the question as asked. However, I think the question may be how do I **safely** read this input? – ctrl-alt-delor Aug 10 '22 at 12:03
6

@ctrl-alt-delor This does not answer the question asked. If a file contains a condition to be checked, then using a print statement prints out the condition to be checked, not the result of checking this condition. – Ben Grossmann Aug 10 '22 at 12:06
@Ben have you tried it? – ctrl-alt-delor Aug 10 '22 at 12:16

python - if conditions in string is true

2 Answers2