===========================
Common Python Syntax Errors
===========================

Python syntax is slightly different than other languages, which can cause some confusion.
A dedicated code editor is the easiest way to find and prevent syntax errors. We recommend
using VS Code on your local machine or Google Colab, which runs everything online.
These programs check for syntax errors as you write code. The following are some typical
syntax errors encountered and how to solve them.



IndentationError
""""""""""""""""

If you encounter an ``IndentationError``, you have an extra/missing whitespace in your code.
Code editors make finding troublesome whitespace easier, but the error should also show the
code snippet which threw the error.

- ``IndentationError: expected an indented block`` means some code is missing an indent after a
  class/method/loop deceleration.

- ``IndentationError: unindent does not match any outer indentation level`` means the code didn't
  return to a previous indentation level.

- ``IndentationError: unexpected indent`` means Python encountered unexpected whitespace.

Code blocks in Python rely on indentation levels (1 level = 4 spaces), so whitespace can't be placed
randomly. Code blocks are preceded by a ``:``, and all code in one block has the same indentation.
To get out of a code block, remove an indentation level.

As an example of indentation, here is some code that adds the numbers 0 to 9:


.. code-block:: python

    # Add numbers 0 through 9
    total = 0
    for i in range(10):
        # New code block (4 spaces)
        total += i
    # Exit loop code block (0 spaces)
    print(total)

Mismatched Brackets and Square Brackets
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Nesting many lists and dictionaries inside each other become hard to read. If you have mismatched or
missing brackets, Python will throw ``SyntaxError: invalid syntax``. Code editors can automatically
format large nestings and highlight which openings and closings go together, making the code easier
to understand.

Make sure all brackets are balanced and that opening and closing brackets match. Python uses
three types of brackets:



* ``()`` is used when creating a `tuple <https://docs.python.org/3/library/stdtypes.html#tuple>`__ or when creating/calling method signatures.
* ``[]`` is used when creating a `list <https://docs.python.org/3/library/stdtypes.html#list>`__ or when indexing an item in a list or tuple.
* ``{}`` is used when creating a `dict <https://docs.python.org/3/library/stdtypes.html#mapping-types-dict>`__ or `set <https://docs.python.org/3/library/stdtypes.html#set>`__.

TypeError: object is not callable
"""""""""""""""""""""""""""""""""

The most common reason ``TypeError: object is not callable`` is when ``()`` is used instead of
``[]``. Parentheses are used to call functions. For example

.. code-block:: python

    def foo(n):
        print("I received", n)


    foo(1)
    # I received 1

But parentheses aren't valid for indexing a subscriptable object (list, tuple, etc.).
For example, the following code will throw a TypeError

.. skip: next

.. code-block:: python

    bar = [1, 2, 3, 4]
    bar(1)

.. code-block:: shell

    Traceback (most recent call last):
      File "<stdin>", line 1, in <module>
    TypeError: 'list' object is not callable

but the following code is valid

.. code-block:: python

    bar = [1, 2, 3, 4]
    print(bar[1])
    # 2

The same applies to dictionaries, but instead of indexing with an integer, you would index
with a keyword. For example

.. skip: next

.. code-block:: python

    spam = {"ham": "Hello World!", "eggs": 54.73}
    print(spam["ham"])  # prints Hello World!
    print(spam("ham"))  # throws error

.. code-block:: shell

    Traceback (most recent call last):
      File "<stdin>", line 1, in <module>
    TypeError: 'dict' object is not callable

TypeError: object is not subscriptable
""""""""""""""""""""""""""""""""""""""

``TypeError: object is not subscriptable`` is thrown when indexing a non-subscriptable object.
For example

.. skip: next

.. code-block:: python

    some_num = 42
    some_num[3]

.. skip: next

.. code-block:: shell

    Traceback (most recent call last):
      File "<stdin>", line 1, in <module>
    TypeError: 'int' object is not subscriptable

Also, there is a limit to how times you can index a subscriptable object. A 1D list can only be indexed once,
2D twice, and so on. If you are using nested lists/dicts, make sure you aren't exceeding the number
of indexes possible.