Introducing Python

Overview

Questions

• What is Python?
• How do we assign variables in Python?
• How do we perform actions in Python?
• How does Python handle different types of data?

Objectives

• Introduce some Python terminology
• Learn how to assign values to variables
• Learn how to use functions and methods
• Introduce the built-in Python data types and functions
• Introduce the Python Standard Library

About Python

---

Python is a high-level, general purpose language. In practical terms, that means it dispenses with many of the complexities of writing code. It is widely used and has an active community of developers, making it suitable for a large number of tasks.

A core philosophical tenet of Python is that code is read more often than it is written. Best practices for coding in Python therefore emphasize writing well-documented, consistently formatted code that follows language-wide standards. In a Pythonic world, everyone’s code would look the same.

One feature to be aware of going in is that Python uses whitespace (as opposed to braces) to organize blocks of code. This can be a sticking point for people used to other languages.

Getting started

---

Everything in Python is an object. Each object has a type, which controls what the object can do. For example, we can add integers together or append a string to a list. An object can be assigned to a variable, and a function performs an action.

Python is an object-oriented language. This means that each object includes built-in variables and functions that can be used from the object itself. When working from an object, we use different terms to refer to these concepts:

• an attribute stores a piece of data
• a method performs an action

Let’s look at one of the main built-in types, str, to see how this works in practice. In Python, str, short for string, is used to store and manipulate text. To get started, we’ll assign the string “hello world” to the variable text. In Python, we use a single equal sign for assignment and quotes to create a string.

Using quotes

Either single or double quotes can be used to create strings, but try to use them consistently! We will use double quotes in the examples here.

First we’ll define a variable called text. Variables can be any type of object supported by Python, but in this case we’ll use a string:

PYTHON

text = "hello world"

Now, instead of retyping the full string every time we need it, we can access that string by referring to the variable:

PYTHON

text

OUTPUT

'hello world'

We can check the type of the variable using the type() function:

PYTHON

type(text)

OUTPUT

str

We can modify the text by calling one of the methods built into the str class. We call methods by appending a period and the name of the method to the object. For example, to display the string in upper case, use upper():

PYTHON

text.upper()

OUTPUT

'HELLO WORLD'

After running a cell, Jupyter displays a representation of the object that appears in the last line of a cell (but note that some actions, like assigning an object, have no output). Some cells will have more than one line, and it is often useful to display content produced earlier in a cell. This can be accomplished using the built-in print() function. In the cell below, we’ll use print() to show a series of strings:

PYTHON

"hello"
print("h")
print("e")
print("l")
print("l")
print("o")
"world"

OUTPUT

h
e
l
l
o

'world'

Note that the string “hello” at the top of the cell lacks a print statement and does not appear in the output, whereas the text “world” does appear in the output because it is the last line of the cell.

Each object may contain many attributes and methods. Use the help() function on any object, including functions or methods, to show a description of the object and list the available methods.

PYTHON

help(str)

OUTPUT

Help on class str in module builtins:

class str(object)
 |  str(object='') -> str
 |  str(bytes_or_buffer[, encoding[, errors]]) -> str
 |
 |  Create a new string object from the given object. If encoding or
 |  errors is specified, then the object must expose a data buffer
 |  that will be decoded using the given encoding and error handler.
 |  Otherwise, returns the result of object.__str__() (if defined)
 |  or repr(object).
 |  encoding defaults to sys.getdefaultencoding().
 |  errors defaults to 'strict'.
 |
 |  Methods defined here:
 |
 |  __add__(self, value, /)
 |      Return self+value.
 |
 |  __contains__(self, key, /)
 |      Return key in self.
 |
 |  __eq__(self, value, /)
 |      Return self==value.
 |
 |  __format__(self, format_spec, /)
 |      Return a formatted version of the string as described by format_spec.
 |
 |  __ge__(self, value, /)
 |      Return self>=value.
 |
 |  __getattribute__(self, name, /)
 |      Return getattr(self, name).
 |
 |  __getitem__(self, key, /)
 |      Return self[key].
 |
 |  __getnewargs__(...)
 |
 |  __gt__(self, value, /)
 |      Return self>value.
 |
 |  __hash__(self, /)
 |      Return hash(self).
 |
 |  __iter__(self, /)
 |      Implement iter(self).
 |
 |  __le__(self, value, /)
 |      Return self<=value.
 |
 |  __len__(self, /)
 |      Return len(self).
 |
 |  __lt__(self, value, /)
 |      Return self<value.
 |
 |  __mod__(self, value, /)
 |      Return self%value.
 |
 |  __mul__(self, value, /)
 |      Return self*value.
 |
 |  __ne__(self, value, /)
 |      Return self!=value.
 |
 |  __repr__(self, /)
 |      Return repr(self).
 |
 |  __rmod__(self, value, /)
 |      Return value%self.
 |
 |  __rmul__(self, value, /)
 |      Return value*self.
 |
 |  __sizeof__(self, /)
 |      Return the size of the string in memory, in bytes.
 |
 |  __str__(self, /)
 |      Return str(self).
 |
 |  capitalize(self, /)
 |      Return a capitalized version of the string.
 |
 |      More specifically, make the first character have upper case and the rest lower
 |      case.
 |
 |  casefold(self, /)
 |      Return a version of the string suitable for caseless comparisons.
 |
 |  center(self, width, fillchar=' ', /)
 |      Return a centered string of length width.
 |
 |      Padding is done using the specified fill character (default is a space).
 |
 |  count(...)
 |      S.count(sub[, start[, end]]) -> int
 |
 |      Return the number of non-overlapping occurrences of substring sub in
 |      string S[start:end].  Optional arguments start and end are
 |      interpreted as in slice notation.
 |
 |  encode(self, /, encoding='utf-8', errors='strict')
 |      Encode the string using the codec registered for encoding.
 |
 |      encoding
 |        The encoding in which to encode the string.
 |      errors
 |        The error handling scheme to use for encoding errors.
 |        The default is 'strict' meaning that encoding errors raise a
 |        UnicodeEncodeError.  Other possible values are 'ignore', 'replace' and
 |        'xmlcharrefreplace' as well as any other name registered with
 |        codecs.register_error that can handle UnicodeEncodeErrors.
 |
 |  endswith(...)
 |      S.endswith(suffix[, start[, end]]) -> bool
 |
 |      Return True if S ends with the specified suffix, False otherwise.
 |      With optional start, test S beginning at that position.
 |      With optional end, stop comparing S at that position.
 |      suffix can also be a tuple of strings to try.
 |
 |  expandtabs(self, /, tabsize=8)
 |      Return a copy where all tab characters are expanded using spaces.
 |
 |      If tabsize is not given, a tab size of 8 characters is assumed.
 |
 |  find(...)
 |      S.find(sub[, start[, end]]) -> int
 |
 |      Return the lowest index in S where substring sub is found,
 |      such that sub is contained within S[start:end].  Optional
 |      arguments start and end are interpreted as in slice notation.
 |
 |      Return -1 on failure.
 |
 |  format(...)
 |      S.format(*args, **kwargs) -> str
 |
 |      Return a formatted version of S, using substitutions from args and kwargs.
 |      The substitutions are identified by braces ('{' and '}').
 |
 |  format_map(...)
 |      S.format_map(mapping) -> str
 |
 |      Return a formatted version of S, using substitutions from mapping.
 |      The substitutions are identified by braces ('{' and '}').
 |
 |  index(...)
 |      S.index(sub[, start[, end]]) -> int
 |
 |      Return the lowest index in S where substring sub is found,
 |      such that sub is contained within S[start:end].  Optional
 |      arguments start and end are interpreted as in slice notation.
 |
 |      Raises ValueError when the substring is not found.
 |
 |  isalnum(self, /)
 |      Return True if the string is an alpha-numeric string, False otherwise.
 |
 |      A string is alpha-numeric if all characters in the string are alpha-numeric and
 |      there is at least one character in the string.
 |
 |  isalpha(self, /)
 |      Return True if the string is an alphabetic string, False otherwise.
 |
 |      A string is alphabetic if all characters in the string are alphabetic and there
 |      is at least one character in the string.
 |
 |  isascii(self, /)
 |      Return True if all characters in the string are ASCII, False otherwise.
 |
 |      ASCII characters have code points in the range U+0000-U+007F.
 |      Empty string is ASCII too.
 |
 |  isdecimal(self, /)
 |      Return True if the string is a decimal string, False otherwise.
 |
 |      A string is a decimal string if all characters in the string are decimal and
 |      there is at least one character in the string.
 |
 |  isdigit(self, /)
 |      Return True if the string is a digit string, False otherwise.
 |
 |      A string is a digit string if all characters in the string are digits and there
 |      is at least one character in the string.
 |
 |  isidentifier(self, /)
 |      Return True if the string is a valid Python identifier, False otherwise.
 |
 |      Call keyword.iskeyword(s) to test whether string s is a reserved identifier,
 |      such as "def" or "class".
 |
 |  islower(self, /)
 |      Return True if the string is a lowercase string, False otherwise.
 |
 |      A string is lowercase if all cased characters in the string are lowercase and
 |      there is at least one cased character in the string.
 |
 |  isnumeric(self, /)
 |      Return True if the string is a numeric string, False otherwise.
 |
 |      A string is numeric if all characters in the string are numeric and there is at
 |      least one character in the string.
 |
 |  isprintable(self, /)
 |      Return True if the string is printable, False otherwise.
 |
 |      A string is printable if all of its characters are considered printable in
 |      repr() or if it is empty.
 |
 |  isspace(self, /)
 |      Return True if the string is a whitespace string, False otherwise.
 |
 |      A string is whitespace if all characters in the string are whitespace and there
 |      is at least one character in the string.
 |
 |  istitle(self, /)
 |      Return True if the string is a title-cased string, False otherwise.
 |
 |      In a title-cased string, upper- and title-case characters may only
 |      follow uncased characters and lowercase characters only cased ones.
 |
 |  isupper(self, /)
 |      Return True if the string is an uppercase string, False otherwise.
 |
 |      A string is uppercase if all cased characters in the string are uppercase and
 |      there is at least one cased character in the string.
 |
 |  join(self, iterable, /)
 |      Concatenate any number of strings.
 |
 |      The string whose method is called is inserted in between each given string.
 |      The result is returned as a new string.
 |
 |      Example: '.'.join(['ab', 'pq', 'rs']) -> 'ab.pq.rs'
 |
 |  ljust(self, width, fillchar=' ', /)
 |      Return a left-justified string of length width.
 |
 |      Padding is done using the specified fill character (default is a space).
 |
 |  lower(self, /)
 |      Return a copy of the string converted to lowercase.
 |
 |  lstrip(self, chars=None, /)
 |      Return a copy of the string with leading whitespace removed.
 |
 |      If chars is given and not None, remove characters in chars instead.
 |
 |  partition(self, sep, /)
 |      Partition the string into three parts using the given separator.
 |
 |      This will search for the separator in the string.  If the separator is found,
 |      returns a 3-tuple containing the part before the separator, the separator
 |      itself, and the part after it.
 |
 |      If the separator is not found, returns a 3-tuple containing the original string
 |      and two empty strings.
 |
 |  removeprefix(self, prefix, /)
 |      Return a str with the given prefix string removed if present.
 |
 |      If the string starts with the prefix string, return string[len(prefix):].
 |      Otherwise, return a copy of the original string.
 |
 |  removesuffix(self, suffix, /)
 |      Return a str with the given suffix string removed if present.
 |
 |      If the string ends with the suffix string and that suffix is not empty,
 |      return string[:-len(suffix)]. Otherwise, return a copy of the original
 |      string.
 |
 |  replace(self, old, new, count=-1, /)
 |      Return a copy with all occurrences of substring old replaced by new.
 |
 |        count
 |          Maximum number of occurrences to replace.
 |          -1 (the default value) means replace all occurrences.
 |
 |      If the optional argument count is given, only the first count occurrences are
 |      replaced.
 |
 |  rfind(...)
 |      S.rfind(sub[, start[, end]]) -> int
 |
 |      Return the highest index in S where substring sub is found,
 |      such that sub is contained within S[start:end].  Optional
 |      arguments start and end are interpreted as in slice notation.
 |
 |      Return -1 on failure.
 |
 |  rindex(...)
 |      S.rindex(sub[, start[, end]]) -> int
 |
 |      Return the highest index in S where substring sub is found,
 |      such that sub is contained within S[start:end].  Optional
 |      arguments start and end are interpreted as in slice notation.
 |
 |      Raises ValueError when the substring is not found.
 |
 |  rjust(self, width, fillchar=' ', /)
 |      Return a right-justified string of length width.
 |
 |      Padding is done using the specified fill character (default is a space).
 |
 |  rpartition(self, sep, /)
 |      Partition the string into three parts using the given separator.
 |
 |      This will search for the separator in the string, starting at the end. If
 |      the separator is found, returns a 3-tuple containing the part before the
 |      separator, the separator itself, and the part after it.
 |
 |      If the separator is not found, returns a 3-tuple containing two empty strings
 |      and the original string.
 |
 |  rsplit(self, /, sep=None, maxsplit=-1)
 |      Return a list of the words in the string, using sep as the delimiter string.
 |
 |        sep
 |          The delimiter according which to split the string.
 |          None (the default value) means split according to any whitespace,
 |          and discard empty strings from the result.
 |        maxsplit
 |          Maximum number of splits to do.
 |          -1 (the default value) means no limit.
 |
 |      Splits are done starting at the end of the string and working to the front.
 |
 |  rstrip(self, chars=None, /)
 |      Return a copy of the string with trailing whitespace removed.
 |
 |      If chars is given and not None, remove characters in chars instead.
 |
 |  split(self, /, sep=None, maxsplit=-1)
 |      Return a list of the words in the string, using sep as the delimiter string.
 |
 |      sep
 |        The delimiter according which to split the string.
 |        None (the default value) means split according to any whitespace,
 |        and discard empty strings from the result.
 |      maxsplit
 |        Maximum number of splits to do.
 |        -1 (the default value) means no limit.
 |
 |  splitlines(self, /, keepends=False)
 |      Return a list of the lines in the string, breaking at line boundaries.
 |
 |      Line breaks are not included in the resulting list unless keepends is given and
 |      true.
 |
 |  startswith(...)
 |      S.startswith(prefix[, start[, end]]) -> bool
 |
 |      Return True if S starts with the specified prefix, False otherwise.
 |      With optional start, test S beginning at that position.
 |      With optional end, stop comparing S at that position.
 |      prefix can also be a tuple of strings to try.
 |
 |  strip(self, chars=None, /)
 |      Return a copy of the string with leading and trailing whitespace removed.
 |
 |      If chars is given and not None, remove characters in chars instead.
 |
 |  swapcase(self, /)
 |      Convert uppercase characters to lowercase and lowercase characters to uppercase.
 |
 |  title(self, /)
 |      Return a version of the string where each word is titlecased.
 |
 |      More specifically, words start with uppercased characters and all remaining
 |      cased characters have lower case.
 |
 |  translate(self, table, /)
 |      Replace each character in the string using the given translation table.
 |
 |        table
 |          Translation table, which must be a mapping of Unicode ordinals to
 |          Unicode ordinals, strings, or None.
 |
 |      The table must implement lookup/indexing via __getitem__, for instance a
 |      dictionary or list.  If this operation raises LookupError, the character is
 |      left untouched.  Characters mapped to None are deleted.
 |
 |  upper(self, /)
 |      Return a copy of the string converted to uppercase.
 |
 |  zfill(self, width, /)
 |      Pad a numeric string with zeros on the left, to fill a field of the given width.
 |
 |      The string is never truncated.
 |
 |  ----------------------------------------------------------------------
 |  Static methods defined here:
 |
 |  __new__(*args, **kwargs) from builtins.type
 |      Create and return a new object.  See help(type) for accurate signature.
 |
 |  maketrans(...)
 |      Return a translation table usable for str.translate().
 |
 |      If there is only one argument, it must be a dictionary mapping Unicode
 |      ordinals (integers) or characters to Unicode ordinals, strings or None.
 |      Character keys will be then converted to ordinals.
 |      If there are two arguments, they must be strings of equal length, and
 |      in the resulting dictionary, each character in x will be mapped to the
 |      character at the same position in y. If there is a third argument, it
 |      must be a string, whose characters will be mapped to None in the result.

Data types

Python also includes data types for representing other types of data, including numbers or collections of data. The core Python data types are introduced in the table below. We’ll talk more about some of these as we encounter them in the lesson:

Type	Definition	Example
str	Character string	"hello world"
int	Integer numerical	42
float	Approximate numerical	0.406
bool	Stores True or False values	True or False
list	Sequence that can be modified	["a", "b", "c"]
tuple	Sequence that cannot be modified	("a", "b", "c")
dict	Mapping of keys to values	{"DC": "District of Columbia", "MD": "Maryland", "VA": "Virginia"}
set	Collection of unique values	{"1", "2", 1}

Libraries

---

Python includes a number of built-in functions that are available wherever Python is installed. See the table below for some examples.

Examples of built-in functions

Name	Description	Example
abs()	Gets the absolute value of a number	abs(-1.5) # returns 1.5
max()	Gets the highest value in a sequence	min([1, 2, 3]) # returns 3
min()	Gets the lowest value in a sequence	min([1, 2, 3]) # returns 1
round()	Rounds a number to the nearest integer	round(5.4) # returns 5

Python also includes a number of built-in libraries. A library bundles functions and other code related to a single task or data type. They are used to simplify the performance of common tasks. By using a common code base, a library allows coders to work more quickly and with fewer errors.

The libraries built into Python are referred to as the Python Standard Library. They can be accessed through a typical Python installation and do not require any additional downloads. A few examples are included in the table below, but as with the table of built-in functions, there are more where those came from.

Examples from the Python Standard Library

Library	Description
datetime	Reads, writes, and analyzes dates and times
os	Create, manipulate, and get information about files and paths
random	Generate pseudo-random numbers

Documentation

The documentation for each library can be viewed by clicking the link in the table. Documentation is an invaluable resource. It provides descriptions of what a library does and detailed information about how it can be used, often including examples.

Unlike the built-in functions, we must import a library before we can use it. We do so using the import statement:

PYTHON

import datetime

Once imported, the library is available for use anywhere in the current document. When using a library, we must include the name of the library to access its functions. For example, to create a datetime.date object (that is, a date object defined by the datetime library), we include both the library and method name:

PYTHON

date = datetime.date(1970, 1, 1)

As with the built-in types, the datetime.date object includes its own suite of attributes and methods. We can, for example, use the year attribute to get the year:

PYTHON

date.year

OUTPUT

1970

Or convert the date to a string using the strftime() method and date format codes. Here, %Y corresponds to YYYY, %m to MM, and %d to DD.

PYTHON

date.strftime("%Y-%m-%d")

OUTPUT

'1970-01-01'

Like Python itself, the Python Standard Library is maintained by the Python Software Foundation. The built-in libraries are limited to a relatively small set of operations expected to be useful to a broad population of users. However, Python allows users to create their own libraries to perform actions that are beyond the scope of core Python. The rest of this lesson will focus on an external library called pandas.

Key Points

• Python is a widely used language that can be used for a variety of tasks, including analyzing data
• Python uses different data types to handle text, numbers, collections, and other kinds of data
• Assign values to variables using the = operator
• Use functions and methods to perform specific actions
• Python’s functionality can be extended using libraries, including libraries written by members of the community that address discipline-specific needs
• Use the help() function and developer documentation to learn more about Python and Python libraries