Everyone who writes Python code encounters error messages on a regular basis—experts included. An error is not a sign of failure; it is Python explaining, as precisely as possible, what went wrong and where execution stopped.
This appendix collects the exceptions and pitfalls that researchers are most likely to encounter in empirical analysis, what each message means, and how to fix it. The examples below are live: each was executed when compiling the book, so the tracebacks match exactly what your Python environment will display.
When Python cannot execute an instruction, it raises an exception and prints a traceback.
KeyError: 'age_years').analysis.py, line 24). That is the exact line where execution halted.SettingWithCopyWarning or FutureWarning) allows the script to continue running, but alerts you to a potential issue or deprecated syntax.NameError: name '...' is not defined--------------------------------------------------------------------------- NameError Traceback (most recent call last) Cell In[2], line 1 ----> 1 average = sum(sleep_hours) / len(sleep_hours) NameError: name 'sleep_hours' is not defined
What it means: Python searched your computer’s memory for an object or variable named sleep_hours and found nothing.
Common causes: - Typing mistake: Variable names in Python are strictly case-sensitive. Sleep_hours and sleep_hours are two completely different names. - Unexecuted line: In an interactive session or notebook, the cell where the variable was defined was not run, or was run in a previous session that has since been restarted. - Missing quotes around text: Writing faculty = Education instead of faculty = "Education" causes Python to look for a variable named Education.
AttributeError: '...' object has no attribute '...'--------------------------------------------------------------------------- AttributeError Traceback (most recent call last) Cell In[3], line 2 1 scores = [78, 85, 92, 64, 88] ----> 2 scores.mean() AttributeError: 'list' object has no attribute 'mean'
What it means: You attempted to call a method or access a property that does not exist on that object type. Here, standard Python list objects have methods like .append() and .sort(), but no .mean() method.
How to fix it: - If working with a list, use Python’s built-in sum(scores) / len(scores) or numpy’s np.mean(scores). - If you intended to use pandas methods, convert the collection to a pandas Series: pd.Series(scores).mean(). - Check spelling: writing df.val_counts() instead of df.value_counts().
KeyError: '...'--------------------------------------------------------------------------- KeyError Traceback (most recent call last) File /opt/hostedtoolcache/Python/3.12.15/x64/lib/python3.12/site-packages/pandas/core/indexes/base.py:3816, in Index.get_loc(self, key) 3815 try: -> 3816 return self._engine.get_loc(casted_key) 3817 except KeyError as err: File pandas/_libs/index.pyx:172, in pandas._libs.index.IndexEngine.get_loc() --> 172 'Could not get source, probably due dynamically evaluated source code.' File pandas/_libs/index.pyx:201, in pandas._libs.index.IndexEngine.get_loc() --> 201 'Could not get source, probably due dynamically evaluated source code.' File pandas/_libs/hashtable_class_helper.pxi:7726, in pandas._libs.hashtable.PyObjectHashTable.get_item() -> 7726 'Could not get source, probably due dynamically evaluated source code.' File pandas/_libs/hashtable_class_helper.pxi:7734, in pandas._libs.hashtable.PyObjectHashTable.get_item() -> 7734 'Could not get source, probably due dynamically evaluated source code.' KeyError: 'faculty_name' The above exception was the direct cause of the following exception: KeyError Traceback (most recent call last) Cell In[4], line 1 ----> 1 students["faculty_name"] File /opt/hostedtoolcache/Python/3.12.15/x64/lib/python3.12/site-packages/pandas/core/frame.py:4330, in DataFrame.__getitem__(self, key) 4326 4327 if is_single_key: 4328 if self.columns.nlevels > 1: 4329 return self._getitem_multilevel(key) -> 4330 indexer = self.columns.get_loc(key) 4331 if is_integer(indexer): 4332 indexer = [indexer] # type: ignore[assignment] 4333 else: File /opt/hostedtoolcache/Python/3.12.15/x64/lib/python3.12/site-packages/pandas/core/indexes/base.py:3823, in Index.get_loc(self, key) 3818 if isinstance(casted_key, slice) or ( 3819 isinstance(casted_key, abc.Iterable) 3820 and any(isinstance(x, slice) for x in casted_key) 3821 ): 3822 raise InvalidIndexError(key) from err -> 3823 raise KeyError(key) from err 3824 except TypeError: 3825 # If we have a listlike key, _check_indexing_error will raise 3826 # InvalidIndexError. Otherwise we fall through and re-raise 3827 # the TypeError. 3828 self._check_indexing_error(key) KeyError: 'faculty_name'
What it means: In pandas or dictionaries, you requested a column name or key that does not exist.
How to fix it: - Check the exact spelling and casing of your DataFrame columns by running list(students.columns). - In the wellbeing dataset, the column is named faculty, not faculty_name. - Check for hidden whitespace in column names (e.g. 'age ' instead of 'age'), which often occurs when importing messy Excel or survey exports. Clean them with df.columns = df.columns.str.strip().
IndexError: list index out of range--------------------------------------------------------------------------- IndexError Traceback (most recent call last) Cell In[5], line 2 1 faculties = ["Health Sciences", "Education", "Social Sciences"] ----> 2 print(faculties[3]) IndexError: list index out of range
What it means: You attempted to access an item at an index position that does not exist.
How to fix it: - Python uses 0-based indexing. In a list of three elements, the items are at positions 0, 1, and 2. Accessing index 3 asks for a fourth item. - To access the last item of any list or array safely, use negative indexing: faculties[-1].
TypeError: unsupported operand type(s)--------------------------------------------------------------------------- TypeError Traceback (most recent call last) Cell In[6], line 1 ----> 1 total_participants = 600 + " students" TypeError: unsupported operand type(s) for +: 'int' and 'str'
What it means: You attempted an operation on incompatible data types—here, adding a number (int) and text (str).
How to fix it: - If you want to combine numbers and text for reporting, convert the number to a string using an f-string: python f"{600} students" - If a numerical column was imported as strings (for example, containing numbers mixed with "N/A"), convert the column with pd.to_numeric(df['col'], errors='coerce').
ValueError: could not convert string to float--------------------------------------------------------------------------- ValueError Traceback (most recent call last) Cell In[7], line 1 ----> 1 float("29.5 years") ValueError: could not convert string to float: '29.5 years'
What it means: A function expected data in a specific format, but the received value could not be parsed. float() cannot convert "29.5 years" because of the trailing text characters.
How to fix it: - Strip extraneous text characters before numeric conversion: python float("29.5 years".replace(" years", "")) - In pandas, use regular expression extraction: df['age'].str.extract(r'(\d+\.?\d*)').astype(float).
SyntaxError: invalid syntaxSyntaxError: expected ':'What it means: Python could not parse your code because the grammar rules of the language were violated.
Common causes: - Forgetting the colon : at the end of an if, for, while, or def line. - Unmatched parentheses, brackets, or quotes: opening ( without a matching ). - Using assignment = where a comparison == was required (e.g. if x = 5:).
IndentationError: expected an indented blockIndentationError: expected an indented block after function definitionWhat it means: Unlike R or C++ which use curly braces {} to group code blocks, Python relies strictly on indentation (usually 4 spaces).
How to fix it: - Indent all statements inside functions, loops, and conditional blocks by 4 spaces: python def calculate_bmi(weight_kg, height_m): return weight_kg / (height_m**2) - Never mix tabs and spaces in the same file. In VS Code, ensure your indentation setting is set to “Spaces: 4”.
FileNotFoundError: [Errno 2] No such file or directory--------------------------------------------------------------------------- FileNotFoundError Traceback (most recent call last) Cell In[8], line 1 ----> 1 df = pd.read_csv("data/non_existent_survey.csv") File /opt/hostedtoolcache/Python/3.12.15/x64/lib/python3.12/site-packages/pandas/io/parsers/readers.py:1866, in read_csv(filepath_or_buffer, sep, delimiter, header, names, index_col, usecols, dtype, engine, converters, true_values, false_values, skipinitialspace, skiprows, skipfooter, nrows, na_values, keep_default_na, na_filter, skip_blank_lines, parse_dates, date_format, dayfirst, cache_dates, iterator, chunksize, compression, thousands, decimal, lineterminator, quotechar, quoting, doublequote, escapechar, comment, encoding, encoding_errors, dialect, on_bad_lines, low_memory, memory_map, float_precision, storage_options, dtype_backend) 1853 kwds_defaults = _refine_defaults_read( 1854 dialect, 1855 delimiter, (...) 1862 lineterminator=lineterminator, 1863 ) 1864 kwds.update(kwds_defaults) -> 1866 return _read(filepath_or_buffer, kwds) File /opt/hostedtoolcache/Python/3.12.15/x64/lib/python3.12/site-packages/pandas/io/parsers/readers.py:423, in _read(filepath_or_buffer, kwds) 420 return result 422 # Create the parser. --> 423 parser = TextFileReader(filepath_or_buffer, **kwds) 425 if chunksize or iterator: 426 return parser File /opt/hostedtoolcache/Python/3.12.15/x64/lib/python3.12/site-packages/pandas/io/parsers/readers.py:2721, in TextFileReader.__init__(self, f, engine, **kwds) 2718 self.options["has_index_names"] = kwds["has_index_names"] 2720 self.handles: IOHandles | None = None -> 2721 self._engine = self._make_engine(f, self.engine) File /opt/hostedtoolcache/Python/3.12.15/x64/lib/python3.12/site-packages/pandas/io/parsers/readers.py:2988, in TextFileReader._make_engine(self, f, engine) 2983 is_text = engine in ("python", "python-fwf") or ( 2984 engine == "c" 2985 and self.options.get("encoding", "utf-8") not in (None, "utf-8") 2986 ) 2987 mode = "r" if is_text else "rb" -> 2988 self.handles = get_handle( 2989 f, 2990 mode, 2991 encoding=self.options.get("encoding", None), 2992 compression=self.options.get("compression", None), 2993 memory_map=self.options.get("memory_map", False), 2994 is_text=is_text, 2995 errors=self.options.get("encoding_errors", "strict"), 2996 storage_options=self.options.get("storage_options", None), 2997 ) 2998 assert self.handles is not None 2999 f = self.handles.handle File /opt/hostedtoolcache/Python/3.12.15/x64/lib/python3.12/site-packages/pandas/io/common.py:948, in get_handle(path_or_buf, mode, encoding, compression, memory_map, is_text, errors, storage_options) 939 handle = open( 940 handle, 941 ioargs.mode, (...) 944 newline="", 945 ) 946 else: 947 # Binary mode --> 948 handle = open(handle, ioargs.mode) 949 handles.append(handle) 951 # Convert BytesIO or file objects passed with an encoding FileNotFoundError: [Errno 2] No such file or directory: 'data/non_existent_survey.csv'
What it means: Python searched the file system at the specified path and found no file with that name.
How to fix it: - Check your current working directory by running: ```python import os
print(os.getcwd()) ``- Relative file paths start from the current working directory. If your script is inscripts/and your data is indata/, writing“data/students.csv”will fail if the working directory is already insidescripts/. Use paths relative to the project root or usepathlib.Path(file).parent` for portable path resolution.
ModuleNotFoundError: No module named '...'What it means: You ran import <package>, but Python could not find that library in the active environment.
How to fix it: - Ensure the library is installed in your terminal using pip install <package>. - In VS Code, verify that the active Python interpreter corresponds to the environment where you installed the package (Ctrl+Shift+P -> Python: Select Interpreter).
Some mistakes in data analysis produce no error message at all, but generate incorrect numbers. Watch out for these three pitfalls:
NaN == NaN)In scientific Python, floating-point missing values (NaN) follow the IEEE 754 standard: a missing value is never equal to anything, not even itself:
Writing students[students['age'] == np.nan] will return an empty table! Always use pandas’ .isna() or .notna():
^ vs. **)In Python, the caret symbol ^ is the bitwise XOR operator, not the mathematical power operator:
2 ^ 3 evaluates to: 1
2 ** 3 evaluates to: 8Always use ** for powers (e.g. (1 + r) ** t).
SettingWithCopyWarning)When you filter a DataFrame with sub = df[df['faculty'] == 'Education'] and then try to create a new column with sub['new_col'] = 1, pandas warns you that it cannot tell whether sub is an independent copy or a view into the original DataFrame.
How to fix it: Always make an explicit copy when creating a working subset: