Reading and Writing Data
All the powerful data structures like the Series and the DataFrames would avail to nothing, if the Pandas module wouldn't provide powerful functionalities for reading in and writing out data. It is not only a matter of having a functions for interacting with files. To be useful to data scientists it also needs functions which support the most important data formats like
- Delimiter-separated files, like e.g. csv
- Microsoft Excel files
- HTML
- XML
- JSON
Delimiter-separated Values
Most people take csv files as a synonym for delimter-separated values files. They leave the fact out of account that csv is an acronym for "comma separated values", which is not the case in many situations. Pandas also uses "csv" and contexts, in which "dsv" would be more appropriate.
Delimiter-separated values (DSV) are defined and stored two-dimensional arrays (for example strings) of data by separating the values in each row with delimiter characters defined for this purpose. This way of implementing data is often used in combination of spreadsheet programs, which can read in and write out data as DSV. They are also used as a general data exchange format.
We call a text file a "delimited text file" if it contains text in DSV format.
For example, the file dollar_euro.txt is a delimited text file and uses tabs (\t) as delimiters.
Reading CSV and DSV Files
Pandas offers two ways to read in CSV or DSV files to be precise:
- DataFrame.from_csv
- read_csv
There is no big difference between those two functions, e.g. they have different default values in some cases and read_csv has more paramters. We will focus on read_csv, because DataFrame.from_csv is kept inside Pandas for reasons of backwards compatibility.
import pandas as pd
exchange_rates = pd.read_csv("data1/dollar_euro.txt",
sep="\t")
print(exchange_rates)
As we can see, read_csv used automatically the first line as the names for the columns. It is possible to give other names to the columns. For this purpose, we have to skip the first line by setting the parameter "header" to 0 and we have to assign a list with the column names to the parameter "names":
import pandas as pd
exchange_rates = pd.read_csv("data1/dollar_euro.txt",
sep="\t",
header=0,
names=["year", "min", "max", "days"])
print(exchange_rates)
Exercise
The file "countries_population.csv" is a csv file, containing the population numbers of all countries (July 2014). The delimiter of the file is a space and commas are used to separate groups of thousands in the numbers. The method 'head(n)' of a DataFrame can be used to give out only the first n rows or lines. Read the file into a DataFrame.
Solution:
pop = pd.read_csv("data1/countries_population.csv",
header=None,
names=["Country", "Population"],
index_col=0,
quotechar="'",
sep=" ",
thousands=",")
print(pop.head(5))
Writing csv Files
We can create csv (or dsv) files with the method "to_csv". Before we do this, we will prepare some data to output, which we will write to a file. We have two csv files with population data for various countries. countries_male_population.csv contains the figures of the male populations and countries_female_population.csv correspondingly the numbers for the female populations. We will create a new csv file with the sum:
column_names = ["Country"] + list(range(2002, 2013))
male_pop = pd.read_csv("data1/countries_male_population.csv",
header=None,
index_col=0,
names=column_names)
female_pop = pd.read_csv("data1/countries_female_population.csv",
header=None,
index_col=0,
names=column_names)
population = male_pop + female_pop
population
population.to_csv("data1/countries_total_population.csv")
We want to create a new DataFrame with all the information, i.e. female, male and complete population. This means that we have to introduce an hierarchical index. Before we do it on our DataFrame, we will introduce this problem in a simple example:
import pandas as pd
shop1 = {"foo":{2010:23, 2011:25}, "bar":{2010:13, 2011:29}}
shop2 = {"foo":{2010:223, 2011:225}, "bar":{2010:213, 2011:229}}
shop1 = pd.DataFrame(shop1)
shop2 = pd.DataFrame(shop2)
both_shops = shop1 + shop2
print("Sales of shop1:\n", shop1)
print("\nSales of both shops\n", both_shops)
shops = pd.concat([shop1, shop2], keys=["one", "two"])
shops
We want to swap the hierarchical indices. For this we will use 'swaplevel':
shops.swaplevel()
shops.sort_index(inplace=True)
shops
We will go back to our initial problem with the population figures. We will apply the same steps to those DataFrames:
pop_complete = pd.concat([population.T,
male_pop.T,
female_pop.T],
keys=["total", "male", "female"])
df = pop_complete.swaplevel()
df.sort_index(inplace=True)
df[["Austria", "Australia", "France"]]
df.to_csv("data1/countries_total_population.csv")
Exercise
- Read in the dsv file (csv) bundeslaender.txt. Create a new file with the columns 'land', 'area', 'female', 'male', 'population' and 'density' (inhabitants per square kilometres.
- print out the rows where the area is greater than 30000 and the population is greater than 10000
- Print the rows where the density is greater than 300
lands = pd.read_csv('data1/bundeslaender.txt', sep=" ")
print(lands.columns.values)
# swap the columns of our DataFrame:
lands = lands.reindex(columns=['land', 'area', 'female', 'male'])
lands[:2]
lands.insert(loc=len(lands.columns),
column='population',
value=lands['female'] + lands['male'])
lands[:3]
lands.insert(loc=len(lands.columns),
column='density',
value=(lands['population'] * 1000 / lands['area']).round(0))
lands[:4]
print(lands.loc[(lands.area>30000) & (lands.population>10000)])
Reading and Writing Excel Files
It is also possible to read and write Microsoft Excel files. The Pandas functionalities to read and write Excel files use the modules 'xlrd' and 'openpyxl'. These modules are not automatically installed by Pandas, so you may have to install them manually!
We will use a simple Excel document to demonstrate the reading capabilities of Pandas. The document sales.xls contains two sheets, one called 'week1' and the other one 'week2'.
An Excel file can be read in with the Pandas function "read_excel". This is demonstrated in the following example Python code:
excel_file = pd.ExcelFile("data1/sales.xls")
sheet = pd.read_excel(excel_file)
sheet
The document "sales.xls" contains two sheets, but we only have been able to read in the first one with "read_excel". A complete Excel document, which can consist of an arbitrary number of sheets, can be completely read in like this:
document = {}
for sheet_name in excel_file.sheet_names:
document[sheet_name] = excel_file.parse(sheet_name)
for sheet_name in document:
print("\n" + sheet_name + ":\n", document[sheet_name])