3.3 Syntax Errors

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Incorrectly written command or instruction will cause a failure in execution. For example, indentation (any length) and : (colon) are part of the syntax for the if statement. In the program below, colon : is missing in the first line.

Example: In the code below, colon is missing in the first line of the if condition.

if 2 < 3                       # colon : is missing
  print("It is less than!")
elif 2 > 3:
  print("It is greater!")
else:
  print("Equal!")

  File "<ipython-input-102-e5ff2ebb180c>", line 1
    if 2 < 3                       # colon : is missing
                                                       ^
SyntaxError: invalid syntax

Example: In the code below, indentation is missing in the block statement of the if condition in line 2.

if 2 < 3:
print("It is less than!")   # block statement needs indentation
elif 2 > 3:
  print("It is greater!")
else:
  print("Equal!")

  File "<ipython-input-103-3c294b21dd46>", line 2
    print("It is less than!")   # block statement needs indentation
        ^
IndentationError: expected an indented block

Example: When an undeclared variable is used in a statement, it causes error messages. In the code below, slope was not defined ahead of the if condition.

if slope > 0:                # slope is not defined can not be compared to zero
  print('increasing')
else:
  print('decreasing')

---------------------------------------------------------------------------
NameError                                 Traceback (most recent call last)
<ipython-input-105-942dc7b20bf4> in <module>()
----> 1 if slope > 0:                # slope is not defined can not be compared to zero
      2   print('increasing')
      3 else:
      4   print('decreasing')

NameError: name 'slope' is not defined

Adapted from Python for Introductory Statistics, by Simon Aman (Truman College, City Colleges of Chicago), licensed under CC BY 4.0. Changes were made: reformatted as an accessible XYZ web edition with live in-browser code cells. License: CC-BY-4.0.