Python data types¶

Notes:

Values and Objects¶

We have been working with values, which are pieces of data that a computer program works with, such as a number or text.
We have been assigning a lot of these values (with the assignment operator =) to objects.

pet = 'Fido' 
pet

'Fido'

age = 6
age

Notes:

In the last few sections, you may have had questions like:

“Why are some values in quotation marks while others are not?”,

“What are these square brackets we keep using in our verb arguments?” or

“Why can we take the mean of some columns and not others?” .

We’ll be answering these questions in this module.

Python needs to categorize things to make sense of them.

Let’s start with some basics.

In these cases, pet and age are objects, and Fido and 6 are values.

These objects can be named anything that begins with a letter and is not “special python words” like range, for, if, else, etc. (we will talk about some of those in the next module).

Data Types¶

Here are some data types built-in to the Python language:

Integers - int
Floating-point numbers - float
Strings - str
Booleans -bool
Lists - list
Tuples - tuple
Sets - set
Dictionaries - dict

Notes:

Values, as we saw in our Fido and 6 examples, can be a variety of different things that get classified by Python as data types.

Numerical Data Types¶

Int¶

age = 6 
type(age)

<class 'int'>

Float¶

age = 6.0
type(age)

<class 'float'>

Notes:

You’ve likely noticed that we have not needed to put any quotation marks around number values. Python recognizes numbers as one of two possible types.

Either as an integer called an int type or a floating-point number called a float.

An int value is a whole number that is either positive, negative, or zero.

We can use the verb type() to find out how Python classifies the value that is stored in an object.

6 is of type int.

A float value is a real number in decimal form. That means even if we have a whole number if it contains a decimal point, it is considered a float.

In contrast, 6.0 is if type float.

NaN¶

weather

      month  season  31st
february  winter   NaN
   march  spring  31.0
   march  spring   NaN

nan_value = weather.loc[2,'31st']
nan_value

nan

type(nan_value)

<class 'numpy.float64'>

Notes:

Something you may have noticed in our Pandas dataframes are NaN values.

This stands for Not A Number, and it is a special value used to represent missing data in pandas.

Contrary to its acronym, it is considered a numeric value, specifically of type float!

Ignore the numpy and 64 for now and concentrate on the float classification for now. We will be discussing NumPy more in Module 8.

Unlike other values, NaN cannot be converted to any other type other than a float.

NoneType¶

name_of_bed_monster = None

type(name_of_bed_monster)

<class 'NoneType'>

Notes:

Unlike NaN which is a special float value, NoneType is its own type, with only one possible value, None.

Let’s say we need to save the name of the monster living under my bed in an object. Unfortunately, we have yet to introduce ourselves to him, so we are not sure what his name is. We also need to keep track that we are missing this information.

We’ve seen this data type in our assignments when we replace the None provided with our solution. Since we still need to have the object created for the structure of our assignments, we simply use None to indicate the object exists but is devoid of information.

Booleans¶

The Boolean (bool) type has two values: True and False.

type(False)

<class 'bool'>

weather

      month  season  31st
february  winter   NaN
   march  spring  31.0
   march  spring   NaN

weather['season'] == 'winter'

   True
  False
  False
Name: season, dtype: bool

Notes:

The Boolean type has two values: True and False. As you can see at the top, the value False is of type bool for Boolean.

We have seen this data type when we were filtering our dataframes with conditions.

In pandas, a condition is evaluated and produces a column indicating whether the condition was met or not for each row.

This column is a Boolean type.

Strings¶

Single quotes, e.g., 'Hello'
Double quotes, e.g., "Goodbye"
Triple single quotes, e.g., '''Yesterday'''
Triple double quotes, e.g., """Tomorrow"""

name_of_bed_monster = 'Mike Wazowski'
name_of_bed_monster

'Mike Wazowski'

name_of_bed_monster = "Mike Wazowski"
name_of_bed_monster

'Mike Wazowski'

name_of_bed_monster = """Mike Wazowski"""
name_of_bed_monster

'Mike Wazowski'

Notes:

Text is stored as a data type called a string (str).

We think of a string as a sequence of characters enclosed in some form of quotations.

We’ve been mostly using single quotations for strings up until this point, but you can enclose them with different types of quotation marks, as shown here.

saying = '''Mike Wazowski said: "My name's Mike Wazowski!"'''
saying

'Mike Wazowski said: "My name\'s Mike Wazowski!"'

missing_bed_monster = ''
missing_bed_monster

''

type(missing_bed_monster)

<class 'str'>

Notes:

If the string contains quotation marks or apostrophes, we can use double quotes or triple single quotes, or triple-double quotes to define the string.

What about empty quotations? When we discussed None type, and we didn’t yet know the monster’s name, we could have also set it to an empty string, as shown here. The benefit of this approach is that we’re already setting the type of the object to string, rather than NoneType.

String Verbs¶

There are A variety of different methods to transform strings or extract information from them.

name_of_bed_monster = 'Mike Wazowski'

len():

len(name_of_bed_monster)

.upper():

name_of_bed_monster.upper()

'MIKE WAZOWSKI'

.lower():

name_of_bed_monster.lower()

'mike wazowski'

Notes:

There are A variety of different methods to transform strings or extract information from them. Here are a few of them.

We can obtain the number of characters in a string with len().
We can change the cases to capitals with .upper().
Or change the cases to lower case with .lower().

name_of_bed_monster = 'Mike Wazowski'

name_of_bed_monster.count('k')

name_of_bed_monster.count('K')

Notes:

We can also count the number of times a substring or character is present in a string with .count().

It’s important to note that many of these verbs are case sensitive!

Casting¶

int to float:

number_of_floating_balloons = float(5)
type(number_of_floating_balloons)

<class 'float'>

An int to a str:

number_of_balloon_strings = str(5)
type(number_of_balloon_strings)

<class 'str'>

float to an int (it will round down to the nearest full integer).

number_of_balloons = int(4.99)
number_of_balloons

Notes:

Sometimes we need to explicitly transform an object from one type to another, which is called casting.

We can do this for some types, but not all.

To cast an object to a new type, we use verbs corresponding to the desired type, such as int(), float(), bool() or str().

bool(0)

False

bool(5.0)

True

int(False)

float(True)

1.0

Notes:

We can even convert int and float values to bool values.

Values of 0 or 0.0 are converted to False, and all other numeric are converted to True.

We can convert Boolean values to int and float values where False is 0 or 0.0 respectively and True is 1 or 1.0.

str(True)

'True'

str(None)

'None'

str(3.2)

'3.2'

Notes:

As suspected, the same applies to strings: Most data types can be converted to strings easily.

We don’t even need to use type() here as we can see, both values now have quotations surrounding the value, meaning they are strings.

float('0')

0.0

bool('True')

True

bool('False')

True

bool('0')

True

Notes:

Casting strings into other data types is a bit more problematic.

Sometimes it works correctly.

Other times it does not give us what we expected, as we can see here - the string False is converted to the Boolean True. Be careful when casting types!

bool('Hello')

True

bool('')

False

float('five')

ValueError: could not convert string to float: 'five'

Detailed traceback: 
  File "<string>", line 1, in <module>

Notes:

In fact, only the empty string gets cast to False; every other string is considered True when turned into a Boolean.

Often, we may not be able to cast a string at all.

bool(None)

False

float(None)

TypeError: float() argument must be a string or a number, not 'NoneType'

Detailed traceback: 
  File "<string>", line 1, in <module>

Notes:

These nuisances can be expected with NoneType values as well.

In summary, take care when casting values.

Programming in Python for Data Science

Python data types¶

Values and Objects¶

Data Types¶

Numerical Data Types¶

Int¶

Float¶

NaN¶

NoneType¶

Booleans¶

Strings¶

String Verbs¶

Casting¶

Let’s practice what we learned!¶