Python replacing substring with a loop

Question

I am taking some arbitrary expression like 6 ++++6, or 6+---+++9++5 and need to parse it into its simplest form (so eg 6+6 and 6-9+5) Why does the following code result in an infinite loop? From debugging I can see that the string is being successfully updated, but it's like the condition is not being reevaluated.

while "--" or "+-" or "-+" or "++" in user_input:
        user_input = user_input.replace("--", "+")
        user_input = user_input.replace("+-", "-")
        user_input = user_input.replace("-+", "-")
        user_input = user_input.replace("++", "+")

`while "foo" or "bar" in str:` is evaluated as `while ("foo") or ("bar" in str):`. Since `"foo"` is a truthy value, it's evaluated as `true` and the left or the condition isn't evaluated — Cid, Jun 18 '20 at 10:47

score 2 · Accepted Answer · answered Jun 18 '20 at 10:50

2

You can make use of any, to create a well defined break-condition for your while loop:

replacements = [
    ("--", "+"),
    ("+-", "-"),
    ("-+", "-"),
    ("++", "+")
]

user_input = '6+---+++9++5'
while any(pattern[0] in user_input for pattern in replacements):
    for pattern in replacements:
        user_input = user_input.replace(*pattern)
print(user_input)

Out:

6-9+5

answered Jun 18 '20 at 10:50

Maurice Meyer

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I always love the use of fixed parameters as opposed to writing manually each line. A list of `(objects, methods, values)` is so fancy and practical to call `getattr(obj, method)(value)` on many items at once. – Guimoute Jun 18 '20 at 11:03

Guimoute · Answer 2 · 2020-06-18T10:55:26.277

1

The way your check whether a string is in user_input is wrong because "--" or "+-" or "-+" or "++" in user_input evaluates to something true.

You need to do

while any(string in user_input for string in ("--", "+-", "-+", "++")):
    # Replacements.

edited Jun 18 '20 at 10:55

answered Jun 18 '20 at 10:48

Guimoute

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Python replacing substring with a loop

2 Answers2