Pandas-Extended

pandas_extended

Pandas (Extended)

The Pandas module is Python's fundamental data analytics library and it provides high-performance, easy-to-use data structures and tools for data analysis. Many Excel features are available in Pandas, including creating pivot tables, computing columns based on other columns, etc. Pandas also facilitates grouping rows by column values and joining tables as in SQL. Pandas is a very comprehensive and mature module that can be used for advanced data analytics, so this tutorial presents a basic overview of Pandas' capabilities.

Table of Contents

Let's import Pandas with the usual convention as pd.

In [1]:
# let's suppress warnings, as they can get annoying sometimes
import warnings
warnings.filterwarnings("ignore")

import pandas as pd

Series

A Series is a one-dimensional array-like object containing a sequence of values and an associated array of data labels, called its index.

Creating a Series

The simplest Series is formed from only an array of data.

In [2]:
obj = pd.Series([2,-1,3,5])
obj
Out[2]:
0    2
1   -1
2    3
3    5
dtype: int64

The string representation of a Series displayed interactively shows the index on the left and the values on the right. You can get the array representation and index object of the Series via its values and index attributes, respectively.

In [3]:
obj.values
Out[3]:
array([ 2, -1,  3,  5])
In [4]:
obj.index # Like range(4)
Out[4]:
RangeIndex(start=0, stop=4, step=1)
In [5]:
list(obj.index) # if you want the index as a list of values
Out[5]:
[0, 1, 2, 3]

We may want to create a Series with an index identifying each data point with a label.

In [6]:
obj2 = pd.Series([2,-1,3,5], index=['a', 'b', 'c', 'd'])
obj2
Out[6]:
a    2
b   -1
c    3
d    5
dtype: int64
In [7]:
list(obj2.index)
Out[7]:
['a', 'b', 'c', 'd']

A Series can have a name.

In [8]:
obj_withName = pd.Series([83, 68], index=["bob", "alice"], name="weights")
obj_withName
Out[8]:
bob      83
alice    68
Name: weights, dtype: int64

You can also create a Series object from a dict. The keys will be used as index labels.

In [9]:
weightdata = {"john": 86, "michael": 68, "alice": 68, "bob": 83}
obj3 = pd.Series(weightdata)
obj3
Out[9]:
john       86
michael    68
alice      68
bob        83
dtype: int64

Selecting and filtering in Series

Compared with NumPy arrays, you can use labels in the index when selecting single values or a set of values.

In [10]:
obj2
Out[10]:
a    2
b   -1
c    3
d    5
dtype: int64
In [11]:
obj2['a']
Out[11]:
2
In [12]:
obj2[['b','c','d']] 
# Here ['b', 'c', 'd'] is interpreted as a list of indices, even though it contains strings instead of integers.
Out[12]:
b   -1
c    3
d    5
dtype: int64

You can still access the items by integer location, like in a regular array. By default, the rank of the item in the Series starts at 0.

In [13]:
obj2[0]
Out[13]:
2
In [14]:
obj2[[1,3]]
Out[14]:
b   -1
d    5
dtype: int64
In [15]:
obj2[obj2 < 0]
Out[15]:
b   -1
dtype: int64

Slicing with labels behaves differently than normal Python slicing in that the endpoint is inclusive.

In [16]:
obj2['b':'c']
Out[16]:
b   -1
c    3
dtype: int64

To make it clear when you are accessing by label or by integer location, it is recommended to always use the loc attribute when accessing by label, and the iloc attribute when accessing by integer location.

In [17]:
obj2.loc["a"]
Out[17]:
2
In [18]:
obj2.iloc[0]
Out[18]:
2

Slicing a Series also slices the index labels.

In [19]:
obj2.iloc[1:3]
Out[19]:
b   -1
c    3
dtype: int64

This can lead to unexpected results when using the default numeric labels, so be careful:

In [20]:
surprise = pd.Series([1000, 1001, 1002, 1003])
surprise
Out[20]:
0    1000
1    1001
2    1002
3    1003
dtype: int64
In [21]:
surprise_slice = surprise[2:]
surprise_slice
Out[21]:
2    1002
3    1003
dtype: int64

Oh look! The first element has index label 2. The element with index label 0 is absent from the slice.

But remember that you can access elements by integer location using the iloc attribute. This illustrates another reason why it's always better to use loc and iloc to access Series objects.

In [22]:
surprise_slice.iloc[0]
Out[22]:
1002

Operations on Series

Series objects behave much like one-dimensional ndarrays, and you can often pass them as parameters to NumPy functions.

In [23]:
import numpy as np
np.exp(obj2)
Out[23]:
a      7.389056
b      0.367879
c     20.085537
d    148.413159
dtype: float64

Arithmetic operations on Series are also possible, and they apply elementwise, just like for ndarrays.

In [24]:
obj2 + [1000,2000,3000,4000]
Out[24]:
a    1002
b    1999
c    3003
d    4005
dtype: int64

Similar to NumPy, if you add a single number to a Series, that number is added to all items in the Series. This is called broadcasting.

In [25]:
obj2 + 1000
Out[25]:
a    1002
b     999
c    1003
d    1005
dtype: int64

The same is true for all binary operations and even conditional operations.

Plotting a Series

pandas makes it easy to plot Series data using matplotlib. Just import matplotlib and call the plot() method:

In [26]:
%matplotlib inline
import matplotlib.pyplot as plt
%config InlineBackend.figure_format = 'retina'
plt.style.use("ggplot")
###
temperatures = [4.4,5.1,6.1,6.2,6.1,6.1,5.7,5.2,4.7,4.1,3.9,3.5]
s7 = pd.Series(temperatures, name="Temperature")
s7.plot()
plt.show();

DataFrame

A DataFrame represents a rectangular table of data and contains an ordered collection of columns, each of which can be a different value type (numeric, string, boolean, etc.). You can see DataFrames as dict of Series.

Creating a DataFrame

There are many ways to construct a DataFrame, though one of the most common is from a dict of equal-length lists or NumPy arrays.

In [27]:
data = {'state': ['Ohio', 'Ohio', 'Ohio', 'Nevada', 'Nevada', 'Nevada'],
'year': [2000, 2001, 2002, 2001, 2002, 2003],
'pop': [1.5, 1.7, 3.6, 2.4, 2.9, 3.2]}
df = pd.DataFrame(data)
df
Out[27]:
state year pop
0 Ohio 2000 1.5
1 Ohio 2001 1.7
2 Ohio 2002 3.6
3 Nevada 2001 2.4
4 Nevada 2002 2.9
5 Nevada 2003 3.2

For large DataFrames, the head method selects only the first five rows.

In [28]:
df.head()
Out[28]:
state year pop
0 Ohio 2000 1.5
1 Ohio 2001 1.7
2 Ohio 2002 3.6
3 Nevada 2001 2.4
4 Nevada 2002 2.9

You can also create a DataFrame by passing a dict of Series objects.

In [29]:
people_dict = {
    "weight": pd.Series([68, 83, 112], index=["alice", "bob", "charles"]),
    "birthyear": pd.Series([1984, 1985, 1992], index=["bob", "alice", "charles"], name="year"),
    "children": pd.Series([0, 3], index=["charles", "bob"]),
    "hobby": pd.Series(["Biking", "Dancing"], index=["alice", "bob"]),
}
people = pd.DataFrame(people_dict)
people
Out[29]:
weight birthyear children hobby
alice 68 1985 NaN Biking
bob 83 1984 3.0 Dancing
charles 112 1992 0.0 NaN

A few things to note:

  • The Series were automatically aligned based on their index.
  • Missing values are represented as NaN.
  • Series names are ignored (the name "year" was dropped).

If you pass a list of columns and/or index row labels to the DataFrame constructor, it will guarantee that these columns and/or rows will exist, in that order, and no other column/row will exist.

In [30]:
df2 = pd.DataFrame(
        people_dict,
        columns=["birthyear", "weight", "height"],
        index=["bob", "alice", "eugene"]
     )
df2
Out[30]:
birthyear weight height
bob 1984.0 83.0 NaN
alice 1985.0 68.0 NaN
eugene NaN NaN NaN

Another convenient way to create a DataFrame is to pass all the values to the constructor as an ndarray, or a list of lists, and specify the column names and row index labels separately:

In [31]:
values = [
            [1985, np.nan, "Biking",   68],
            [1984, 3,      "Dancing",  83],
            [1992, 0,      np.nan,    112]
         ]
df3 = pd.DataFrame(
        values,
        columns=["birthyear", "children", "hobby", "weight"],
        index=["alice", "bob", "charles"]
     )
df3
Out[31]:
birthyear children hobby weight
alice 1985 NaN Biking 68
bob 1984 3.0 Dancing 83
charles 1992 0.0 NaN 112

You can access columns pretty much as you would expect. They are returned as Series objects.

In [32]:
people["birthyear"]
Out[32]:
alice      1985
bob        1984
charles    1992
Name: birthyear, dtype: int64

You can also get multiple columns at once.

In [33]:
people[["birthyear", "hobby"]]
Out[33]:
birthyear hobby
alice 1985 Biking
bob 1984 Dancing
charles 1992 NaN

To specify missing values, you can either use np.nan or NumPy's masked arrays:

In [34]:
masked_array = np.ma.asarray(values, dtype=np.object)
masked_array[(0, 2), (1, 2)] = np.ma.masked
df3 = pd.DataFrame(
        masked_array,
        columns=["birthyear", "children", "hobby", "weight"],
        index=["alice", "bob", "charles"]
     )
df3
Out[34]:
birthyear children hobby weight
alice 1985 NaN Biking 68
bob 1984 3 Dancing 83
charles 1992 0 NaN 112

Instead of an ndarray, you can also pass a DataFrame object:

In [35]:
df4 = pd.DataFrame(
         df3,
         columns=["hobby", "children"],
         index=["alice", "bob"]
     )
df4
Out[35]:
hobby children
alice Biking NaN
bob Dancing 3

Selecting and filtering in DataFrame

Let's go back to the people:

In [36]:
people
Out[36]:
weight birthyear children hobby
alice 68 1985 NaN Biking
bob 83 1984 3.0 Dancing
charles 112 1992 0.0 NaN

The loc attribute lets you access rows instead of columns. The result is a Series object in which the DataFrame's column names are mapped to row index labels.

In [37]:
people.loc["charles"]
Out[37]:
weight        112
birthyear    1992
children        0
hobby         NaN
Name: charles, dtype: object

You can also access rows by integer location using the iloc attribute.

In [38]:
people.iloc[2]
Out[38]:
weight        112
birthyear    1992
children        0
hobby         NaN
Name: charles, dtype: object

You can also get a slice of rows, and this returns a DataFrame object.

In [39]:
people.iloc[1:3]
Out[39]:
weight birthyear children hobby
bob 83 1984 3.0 Dancing
charles 112 1992 0.0 NaN

Finally, you can pass a boolean array to get the matching rows.

In [40]:
people[np.array([True, False, True])]
Out[40]:
weight birthyear children hobby
alice 68 1985 NaN Biking
charles 112 1992 0.0 NaN

This is most useful when combined with boolean expressions.

In [41]:
people[people["birthyear"] < 1990]
Out[41]:
weight birthyear children hobby
alice 68 1985 NaN Biking
bob 83 1984 3.0 Dancing

Transposing

You can swap columns and indices using the T attribute.

In [42]:
people.T
Out[42]:
alice bob charles
weight 68 83 112
birthyear 1985 1984 1992
children NaN 3 0
hobby Biking Dancing NaN

Adding and removing columns

You can generally treat DataFrame objects like dictionaries of Series, so the following work fine.

In [43]:
people
Out[43]:
weight birthyear children hobby
alice 68 1985 NaN Biking
bob 83 1984 3.0 Dancing
charles 112 1992 0.0 NaN
In [44]:
people["age"] = 2018 - people["birthyear"]  # adds a new column "age"
people["over 30"] = people["age"] > 30      # adds another column "over 30"
birthyears = people.pop("birthyear")
del people["children"]

people
Out[44]:
weight hobby age over 30
alice 68 Biking 33 True
bob 83 Dancing 34 True
charles 112 NaN 26 False
In [45]:
birthyears
Out[45]:
alice      1985
bob        1984
charles    1992
Name: birthyear, dtype: int64

When you add a new colum, it must have the same number of rows. Missing rows are filled with NaN, and extra rows are ignored.

In [46]:
people["pets"] = pd.Series({"bob": 0, "charles": 5, "eugene":1})  # alice is missing, eugene is ignored
people
Out[46]:
weight hobby age over 30 pets
alice 68 Biking 33 True NaN
bob 83 Dancing 34 True 0.0
charles 112 NaN 26 False 5.0

When adding a new column, it is added at the end (on the right) by default. You can also insert a column anywhere else using the insert() method.

In [47]:
people.insert(1, "height", [172, 181, 185])
people
Out[47]:
weight height hobby age over 30 pets
alice 68 172 Biking 33 True NaN
bob 83 181 Dancing 34 True 0.0
charles 112 185 NaN 26 False 5.0

Assigning new columns

You can also create new columns by calling the assign() method. Note that this returns a new DataFrame object, the original is not modified.

In [48]:
people.assign(
    body_mass_index = people["weight"] / (people["height"] / 100) ** 2,
    has_pets = people["pets"] > 0
)
Out[48]:
weight height hobby age over 30 pets body_mass_index has_pets
alice 68 172 Biking 33 True NaN 22.985398 False
bob 83 181 Dancing 34 True 0.0 25.335002 False
charles 112 185 NaN 26 False 5.0 32.724617 True

Note that you cannot access columns created within the same assignment.

In [49]:
try:
    people.assign(
        body_mass_index = people["weight"] / (people["height"] / 100) ** 2,
        overweight = people["body_mass_index"] > 25
    )
except KeyError as e:
    print("Key error:", e)

Key error: 'body_mass_index'

The solution is to split this assignment in two consecutive assignments.

In [50]:
df5 = people.assign(body_mass_index = people["weight"] / (people["height"] / 100) ** 2)
df5.assign(overweight = df5["body_mass_index"] > 25)
Out[50]:
weight height hobby age over 30 pets body_mass_index overweight
alice 68 172 Biking 33 True NaN 22.985398 False
bob 83 181 Dancing 34 True 0.0 25.335002 True
charles 112 185 NaN 26 False 5.0 32.724617 True

Having to create a temporary variable df5 is not very convenient. You may want to just chain the assigment calls, but it does not work because the people object is not actually modified by the first assignment.

In [51]:
try:
    (people
         .assign(body_mass_index = people["weight"] / (people["height"] / 100) ** 2)
         .assign(overweight = people["body_mass_index"] > 25)
    )
except KeyError as e:
    print("Key error:", e)

Key error: 'body_mass_index'

But fear not, there is a simple solution. You can pass a function to the assign() method (typically a lambda function), and this function will be called with the DataFrame as a parameter.

In [52]:
(people
     .assign(body_mass_index = lambda df: df["weight"] / (df["height"] / 100) ** 2)
     .assign(overweight = lambda df: df["body_mass_index"] > 25)
)
Out[52]:
weight height hobby age over 30 pets body_mass_index overweight
alice 68 172 Biking 33 True NaN 22.985398 False
bob 83 181 Dancing 34 True 0.0 25.335002 True
charles 112 185 NaN 26 False 5.0 32.724617 True

Problem solved!

Evaluating an expression

A great feature supported by pandas is expression evaluation. This relies on the numexpr library which must be installed.

In [53]:
people.eval("weight / (height/100) ** 2 > 25")
Out[53]:
alice      False
bob         True
charles     True
dtype: bool

Assignment expressions are also supported. Let's set inplace=True to directly modify the DataFrame rather than getting a modified copy:

In [54]:
people.eval("body_mass_index = weight / (height/100) ** 2", inplace=True)
people
Out[54]:
weight height hobby age over 30 pets body_mass_index
alice 68 172 Biking 33 True NaN 22.985398
bob 83 181 Dancing 34 True 0.0 25.335002
charles 112 185 NaN 26 False 5.0 32.724617

You can use a local or global variable in an expression by prefixing it with '@'.

In [55]:
overweight_threshold = 30
people.eval("overweight = body_mass_index > @overweight_threshold", inplace=True)
people
Out[55]:
weight height hobby age over 30 pets body_mass_index overweight
alice 68 172 Biking 33 True NaN 22.985398 False
bob 83 181 Dancing 34 True 0.0 25.335002 False
charles 112 185 NaN 26 False 5.0 32.724617 True

Querying a DataFrame

The query() method lets you filter a DataFrame based on a query expression.

In [56]:
people.query("age > 30 and pets == 0")
Out[56]:
weight height hobby age over 30 pets body_mass_index overweight
bob 83 181 Dancing 34 True 0.0 25.335002 False

Sorting a DataFrame

You can sort a DataFrame by calling its sort_index method. By default it sorts the rows by their index label, in ascending order, but let's reverse the order.

In [57]:
people.sort_index(ascending=False)
Out[57]:
weight height hobby age over 30 pets body_mass_index overweight
charles 112 185 NaN 26 False 5.0 32.724617 True
bob 83 181 Dancing 34 True 0.0 25.335002 False
alice 68 172 Biking 33 True NaN 22.985398 False

Note that sort_index returned a sorted copy of the DataFrame. To modify people directly, we can set the inplace argument to True. Also, we can sort the columns instead of the rows by setting axis=1.

In [58]:
people.sort_index(axis=1, inplace=True)
people
Out[58]:
age body_mass_index height hobby over 30 overweight pets weight
alice 33 22.985398 172 Biking True False NaN 68
bob 34 25.335002 181 Dancing True False 0.0 83
charles 26 32.724617 185 NaN False True 5.0 112

To sort the DataFrame by the values instead of the labels, we can use sort_values and specify the column to sort by.

In [59]:
people.sort_values(by="age", inplace=True)
people
Out[59]:
age body_mass_index height hobby over 30 overweight pets weight
charles 26 32.724617 185 NaN False True 5.0 112
alice 33 22.985398 172 Biking True False NaN 68
bob 34 25.335002 181 Dancing True False 0.0 83

Operations on DataFrame

Although DataFrames do not try to mimick NumPy arrays, there are a few similarities. Let's create a DataFrame to demonstrate this:

In [60]:
grades_array = np.array([[8,8,9],[10,9,9],[4, 8, 2], [9, 10, 10]])
grades = pd.DataFrame(grades_array, columns=["sep", "oct", "nov"], index=["alice","bob","charles","darwin"])
grades
Out[60]:
sep oct nov
alice 8 8 9
bob 10 9 9
charles 4 8 2
darwin 9 10 10

You can apply NumPy mathematical functions on a DataFrame. The function is applied to all values.

In [61]:
np.sqrt(grades)
Out[61]:
sep oct nov
alice 2.828427 2.828427 3.000000
bob 3.162278 3.000000 3.000000
charles 2.000000 2.828427 1.414214
darwin 3.000000 3.162278 3.162278

Similarly, adding a single value to a DataFrame will add that value to all elements in the DataFrame. This is called broadcasting.

In [62]:
grades + 1
Out[62]:
sep oct nov
alice 9 9 10
bob 11 10 10
charles 5 9 3
darwin 10 11 11

Of course, the same is true for all other binary operations, including arithmetic (*,/,**...) and conditional (>, ==...) operations.

In [63]:
grades >= 5
Out[63]:
sep oct nov
alice True True True
bob True True True
charles False True False
darwin True True True

Aggregation operations, such as computing the max, the sum or the mean of a DataFrame, apply to each column, and you get back a Series object.

In [64]:
grades.mean()
Out[64]:
sep    7.75
oct    8.75
nov    7.50
dtype: float64

The all method is also an aggregation operation: it checks whether all values are True or not. Let's see during which months all students got a grade greater than 5.

In [65]:
(grades > 5).all()
Out[65]:
sep    False
oct     True
nov    False
dtype: bool

Most of these functions take an optional axis parameter which lets you specify along which axis of the DataFrame you want the operation executed. The default is axis=0, meaning that the operation is executed vertically (on each column). You can set axis=1 to execute the operation horizontally (on each row). For example, let's find out which students had all grades greater than 5:

In [66]:
(grades > 5).all(axis = 1)
Out[66]:
alice       True
bob         True
charles    False
darwin      True
dtype: bool

The any method returns True if any value is True. Let's see who got at least one grade 10:

In [67]:
(grades == 10).any(axis = 1)
Out[67]:
alice      False
bob         True
charles    False
darwin      True
dtype: bool

If you add a Series object to a DataFrame (or execute any other binary operation), pandas attempts to broadcast the operation to all rows in the DataFrame. This only works if the Series has the same size as the DataFrames rows. For example, let's substract the mean of the DataFrame (a Series object) from the DataFrame:

In [68]:
grades - grades.mean()  # equivalent to: grades - [7.75, 8.75, 7.50]
Out[68]:
sep oct nov
alice 0.25 -0.75 1.5
bob 2.25 0.25 1.5
charles -3.75 -0.75 -5.5
darwin 1.25 1.25 2.5

We substracted 7.75 from all September grades, 8.75 from October grades and 7.50 from November grades. It is equivalent to substracting this DataFrame.

In [69]:
pd.DataFrame([[7.75, 8.75, 7.50]]*4, index=grades.index, columns=grades.columns)
Out[69]:
sep oct nov
alice 7.75 8.75 7.5
bob 7.75 8.75 7.5
charles 7.75 8.75 7.5
darwin 7.75 8.75 7.5

If you want to substract the global mean from every grade, here is one way to do it:

In [70]:
grades - grades.values.mean() # substracts the global mean (8.00) from all grades
Out[70]:
sep oct nov
alice 0.0 0.0 1.0
bob 2.0 1.0 1.0
charles -4.0 0.0 -6.0
darwin 1.0 2.0 2.0

Plotting a DataFrame

Just like for Series, pandas makes it easy to draw nice graphs based on a DataFrame.

For example, it is easy to create a bar plot from a DataFrame's data by calling its plot method.

In [71]:
people.plot(kind = "bar", y = ["body_mass_index"])
plt.show();

You can pass extra arguments supported by matplotlib's functions. For example, we can create scatterplot and pass it a list of sizes using the s argument of matplotlib's scatter() function.

In [72]:
people.plot(kind = "scatter", x = "height", y = "weight", s=[40, 120, 200])
plt.show();

Again, there are way too many options to list here: the best option is to scroll through the Visualization page in pandas documentation and find the plot you are interested in.

Handling missing data

Dealing with missing data is a frequent task when working with real life data. pandas offers a few tools to handle missing data.

The isnull and notnull functions in pandas can be used to detect missing data.

In [73]:
pd.isnull(grades)
Out[73]:
sep oct nov
alice False False False
bob False False False
charles False False False
darwin False False False
In [74]:
pd.notnull(grades)
Out[74]:
sep oct nov
alice True True True
bob True True True
charles True True True
darwin True True True

It's a bit unfair that we're setting grades to zero in September, though. Perhaps we should decide that missing grades are missing grades, but missing bonus points should be replaced by zeros.

In [ ]:
fixed_bonus_points = bonus_points.fillna(0)
fixed_bonus_points.insert(0, "sep", 0)
fixed_bonus_points.loc["alice"] = 0
grades + fixed_bonus_points

That's much better: although we made up some data, we have not been too unfair.

Another way to handle missing data is to interpolate. Let's look at the bonus_points DataFrame again:

In [ ]:
bonus_points

Now let's call the interpolate method. By default, it interpolates vertically (axis=0), so let's tell it to interpolate horizontally (axis=1).

In [ ]:
bonus_points.interpolate(axis=1)

Bob had 0 bonus points in October, and 2 in December. When we interpolate for November, we get the mean: 1 bonus point. Colin had 1 bonus point in November, but we do not know how many bonus points he had in September, so we cannot interpolate, this is why there is still a missing value in October after interpolation. To fix this, we can set the September bonus points to 0 before interpolation.

In [ ]:
better_bonus_points = bonus_points.copy()
better_bonus_points.insert(0, "sep", 0)
better_bonus_points.loc["alice"] = 0
better_bonus_points = better_bonus_points.interpolate(axis=1)
better_bonus_points

Great, now we have reasonable bonus points everywhere. Let's find out the final grades:

In [ ]:
grades + better_bonus_points

It is slightly annoying that the September column ends up on the right. This is because the DataFrames we are adding do not have the exact same columns (the grades DataFrame is missing the "dec" column), so to make things predictable, pandas orders the final columns alphabetically. To fix this, we can simply add the missing column before adding.

In [ ]:
grades["dec"] = np.nan
final_grades = grades + better_bonus_points
final_grades

There's not much we can do about December and Colin: it's bad enough that we are making up bonus points, but we can't reasonably make up grades (well I guess some teachers probably do). So let's call the dropna() method to get rid of rows that are full of NaNs:

In [ ]:
final_grades_clean = final_grades.dropna(how="all")
final_grades_clean

Now let's remove columns that are full of NaNs by setting the axis argument to 1:

In [ ]:
final_grades_clean = final_grades_clean.dropna(axis=1, how="all")
final_grades_clean

Overview functions

When dealing with large DataFrames, it is useful to get a quick overview of its content. pandas offers a few functions for this. First, let's create a large DataFrame with a mix of numeric values, missing values and text values. Notice how Jupyter displays only the corners of the DataFrame:

In [ ]:
much_data = np.fromfunction(lambda x,y: (x+y*y)%17*11, (10000, 26))
large_df = pd.DataFrame(much_data, columns=list("ABCDEFGHIJKLMNOPQRSTUVWXYZ"))
large_df[large_df % 16 == 0] = np.nan
large_df.insert(3,"some_text", "Blabla")
large_df.head(10)

The head() method returns the top 5 rows.

In [ ]:
large_df.head()

Of course there's also a tail() function to view the bottom 5 rows. You can pass the number of rows you want.

In [ ]:
large_df.tail(n=2)

The info() method prints out a summary of each columns contents.

In [ ]:
large_df.info()

Finally, the describe() method gives a nice overview of the main aggregated values over each column:

  • count: number of non-null (not NaN) values
  • mean: mean of non-null values
  • std: standard deviation of non-null values
  • min: minimum of non-null values
  • 25%, 50%, 75%: 25th, 50th and 75th percentile of non-null values
  • max: maximum of non-null values
In [ ]:
large_df.describe()

Combining DataFrames

Concatenation

Rather than joining DataFrames, we may just want to concatenate them using concat() method.

In [ ]:
result_concat = pd.concat([city_loc, city_pop], sort = False)
result_concat

Note that this operation aligned the data horizontally (by columns) but not vertically (by rows). In this example, we end up with multiple rows having the same index (eg. 3). pandas handles this rather gracefully.

In [ ]:
result_concat.loc[3]

Or you can tell pandas to just ignore the index.

In [ ]:
pd.concat([city_loc, city_pop], ignore_index=True, sort = False)

Notice that when a column does not exist in a DataFrame, it acts as if it was filled with NaN values. If we set join="inner", then only columns that exist in both DataFrames are returned.

In [ ]:
pd.concat([city_loc, city_pop], join="inner")

You can concatenate DataFrames horizontally instead of vertically by setting axis=1.

In [ ]:
pd.concat([city_loc, city_pop], axis=1)

In this case it really does not make much sense because the indices do not align well (eg. Cleveland and San Francisco end up on the same row, because they shared the index label 3). So let's reindex the DataFrames by city name before concatenating:

In [ ]:
pd.concat([city_loc.set_index("city"), city_pop.set_index("city")], axis=1, sort = False)

This looks a lot like a FULL OUTER JOIN, except that the state columns were not renamed to state_x and state_y, and the city column is now the index.

The append() method is a useful shorthand for concatenating DataFrames vertically:

In [ ]:
city_loc.append(city_pop, sort = False)

Categories

As always in pandas, the append() method does not actually modify city_loc: it works on a copy and returns the modified copy.

It is quite frequent to have values that represent categories, for example 1 for female and 2 for male, or "A" for Good, "B" for Average, "C" for Bad. These categorical values can be hard to read and cumbersome to handle, but fortunately pandas makes it easy. To illustrate this, let's take the city_pop DataFrame we created earlier, and add a column that represents a category:

In [ ]:
city_econ = city_pop.copy()
city_econ["econ_code"] = [17, 17, 34, 20]
city_econ

Right now the econ_code column is full of apparently meaningless codes. Let's fix that. First, we will create a new categorical column based on the econ_codes:

In [ ]:
city_econ["economy"] = city_econ["econ_code"].astype('category')
city_econ["economy"].cat.categories

Now we can give each category a meaningful name:

In [ ]:
city_econ["economy"].cat.categories = ["Finance", "Energy", "Tourism"]
city_econ

Note that categorical values are sorted according to their categorical order, not their alphabetical order:

In [ ]:
city_econ.sort_values(by="economy", ascending=False)

Saving and loading

pandas can save DataFrames to various backends, including file formats such as CSV, Excel, JSON, HTML and HDF5, or to a SQL database. Let's create a DataFrame to demonstrate this:

In [ ]:
my_df = pd.DataFrame(
    [["Biking", 68.5, 1985, np.nan], ["Dancing", 83.1, 1984, 3]], 
    columns=["hobby","weight","birthyear","children"],
    index=["alice", "bob"]
)
my_df

Saving

Let's save it as a CSV file.

In [ ]:
my_df.to_csv("my_df.csv")

Loading

Now let's load our CSV file back into a DataFrame:

In [ ]:
my_df_loaded = pd.read_csv("my_df.csv", index_col=0)
my_df_loaded

As you might guess, there are similar read_json, read_html, read_excel functions as well. We can also read data straight from the Internet. For example, let's load all New York State Zip codes from data.ny.gov:

In [ ]:
ny_zip = None
try:
    csv_url = "https://data.ny.gov/api/views/juva-r6g2/rows.csv"
    ny_zip = pd.read_csv(csv_url, index_col=0)
    ny_zip = ny_zip.head()
except IOError as e:
    print(e)
ny_zip

There are more options available, in particular regarding datetime format. Check out the documentation for more details.

Exercises

  1. Create a Series using the following number: 3.14, 2.718, 1.618 with the following labels "pi", "euler's number", "golden ratio". Then filter values that are only greater than 2.
  1. Create the following DataFrame (But use name as the index):
name age state num_children num_pets
john 23 iowa 2 0
mary 78 dc 2 4
peter 22 california 0 0
jeff 19 texas 1 5
bill 45 washington 2 0
lisa 33 dc 1 0

Then, create a bar plot that shows age for each person in name.

  1. Add another person as a new row to the previous DataFrame with the following values (HINT: use pd.concat):

name: mike, age: 0, state: new york, num_children: 1, num_pets: 0.

Since this new person has a child, his age cannot be zero. Replace it with the median age of all other people in the DataFrame.

Possible solutions

  1. Indexing and selecting in Series
s = pd.Series([3.14, 2.718, 1.618], index = ["pi", "euler's number", "golden ratio"])

s[s > 2]
  1. Creating and plotting a DataFrame
df1 = pd.DataFrame(data={'age':[23,78,22,19,45,33],'state':['iowa','dc','california','texas','washington','dc'],'num_children':[2,2,0,1,2,1],'num_pets':[0,4,0,5,0,0]},index=['john','mary','peter','jeff','bill','lisa'])

df1.plot(kind = "bar", y = "age")
  1. Adding new row and handling missing data
df2 = pd.DataFrame(data={'age':[0],'state':['new york'],'num_children':[1],'num_pets':[0]},index=['mike'])
df3 = pd.concat([df1,df2]) 
df2['age'] = df1['age'].median() # Calculate and add the median age of all other people to 'mike'
df3 = pd.concat([df1,df2]) # Update the DataFrame

References

www.featureranking.com