rspec/rules/S3457/python/rule.adoc

Formatting strings, either with the ``++%++`` operator or ``++str.format++`` method, requires a valid string and arguments matching this string's replacement fields.

This also applies to loggers from the ``++logging++`` module. Internally they use ``++%-formatting++``. The only difference is that they will log an error instead of raising an exception when provided arguments are invalid.

Formatted string literals, also called "f-strings", are generally simpler to use, and any syntax mistake will fail at compile time. However it is easy to forget curly braces  and it won't raise any error.

This rule raises an issue when:

* A string formatted with ``++%++`` will not return the expected string because some arguments are not used.
* A string formatted with ``++str.format++`` will not return the expected string because some arguments are not used.
* An "f-string" doesn't contain any replacement field, which probably means that some curly braces are missing.
* Loggers will log an error because their message is not formatted properly.

Rule S2275 covers cases where formatting a string will raise an exception.

== Noncompliant Code Example

----
"Error %(message)s" % {"message": "something failed", "extra": "some dead code"}  # Noncompliant. Remove the unused argument "extra" or add a replacement field.

"Error: User {} has not been able to access []".format("Alice", "MyFile")  # Noncompliant. Remove 1 unexpected argument or add a replacement field.

user = "Alice"
resource = "MyFile"
message = f"Error: User [user] has not been able to access [resource]"  # Noncompliant. Add replacement fields or use a normal string instead of an f-string.

import logging
logging.error("Error: User %s has not been able to access %s", "Alice")  # Noncompliant. Add 1 missing argument.
----

== Compliant Solution

----
"Error %(message)s" % {"message": "something failed"}

"Error: User {} has not been able to access {}".format("Alice", "MyFile")

user = "Alice"
resource = "MyFile"
message = f"Error: User {user} has not been able to access {resource}"

import logging
logging.error("Error: User %s has not been able to access %s", "Alice", "MyFile")
----

== See

* https://docs.python.org/3/library/string.html#format-string-syntax[Python documentation - Format String Syntax]
* https://docs.python.org/3/library/stdtypes.html#printf-style-string-formatting[Python documentation - printf-style String Formatting]
* https://docs.python.org/3/howto/logging.html#loggers[Python documentation - Loggers]
* https://docs.python.org/3/howto/logging-cookbook.html#using-particular-formatting-styles-throughout-your-application[Python documentation - Using particular formatting styles throughout your application]
* https://docs.python.org/3/reference/lexical_analysis.html#formatted-string-literals[Python documentation - Formatted string literals]