Warnings About Dangerous Syntax

In this lesson, you’ll learn about new additions to SyntaxWarning. Python has a SyntaxWarning that can warn you about dubious syntax that is typically not a SyntaxError. Python 3.8 adds a few new ones that can help you when you’re coding and debugging.

The difference between is and == can be confusing. The latter checks for equal values, while is is True only when objects are the same. Python 3.8 will try to warn you about cases when you should use == instead of is:

>>> # Python 3.7
>>> version = "3.7"
>>> version is "3.7"
False

>>> # Python 3.8
>>> version = "3.8"
>>> version is "3.8"
<stdin>:1: SyntaxWarning: "is" with a literal. Did you mean "=="?
False

>>> version == "3.8"
True

It’s easy to miss a comma when you’re writing out a long list, especially when formatting it vertically. Forgetting a comma in a list of tuples will give a confusing error message about tuples not being callable. Python 3.8 additionally emits a warning that points toward the real issue:

>>> [
...   (3, 5)
...   (7, 9)
... ]
<stdin>:2: SyntaxWarning: 'tuple' object is not callable; perhaps
           you missed a comma?
Traceback (most recent call last):
  File "<stdin>", line 2, in <module>
TypeError: 'tuple' object is not callable

The warning correctly identifies the missing comma as the real culprit.

Here are a few resources with info on using IPython, the REPL(Read–Eval–Print Loop) tool used in this video:

• An Effective Python Environment: Making Yourself at Home
• IPython.org