# Python Cheat Sheet for Data Science: Basics

It's common when first learning Python for Data Science to have trouble remembering all the syntax that you need. While at Dataquest we advocate getting used to consulting the Python documentation, sometimes it's nice to have a handy reference, so we've put together this cheat sheet to help you out!

This cheat sheet is the companion to our Python Intermediate Data Science Cheat Sheet

If you'd like to learn Python, we have a Python Programming: Beginner course which can start you on your data science journey.

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## Key Basics, Printing and Getting Help

`x = 3`

| Assign 3 to the variable `x`

`print(x)`

| Print the value of x

`type(x)`

| Return the type of the variable `x`

(in this case, `int`

for integer)

`help(x)`

| Show documentation for the `str`

data type

`help(print)`

| Show documentation for the `print()`

function

## Reading Files

```
f = open("my_file.txt", "r")
file_as_string = f.read()
```

Open the file `my_file.txt`

and assign its contents to `string`

```
import csv
f = open("my_dataset.csv", "r")
csvreader = csv.reader(f)
csv_as_list = list(csvreader)
```

Open the CSV file `my_dataset.csv`

and assign its data to the list of lists `csv_as_list`

## Strings

`s = "hello"`

| Assign the string `"hello"`

to the variable `s`

```
s = """
She said,"there's a good idea.
""""
```

Assign a multi-line string to the variable `s`

. Also used to create strings that contain both" and ' characters.

`len(s)`

| Return the number of characters in `s`

`s.startswith("hel")`

| Test whether `s`

starts with the substring `"hel"`

`s.endswith("lo")`

| Test whether `s`

ends with the substring `"lo"`

`"{} plus {} is {}".format(3,1,4)`

| Return the string with the values `3`

, `1`

, and `4`

inserted

`s.replace("e","z")`

| Return a new string based on `s`

with all occurrences of `"e"`

replaced with `"z"`

`s.strip()`

| Return a new string based off `s`

with any whitespace at the start and end of the string removed

`s.split(" ")`

| Split the string s into a list of strings, separating on the character `" "`

and return that list

## Numeric Types and Mathematical Operations

`i = int("5")`

| Convert the string `"5"`

to the integer `5`

and assign the result to `i`

`f = float("2.5")`

| Convert the string `"2.5"`

to the float value `2.5`

and assign the result to `f`

`5 + 5`

| Addition

`5 - 5`

| Subtraction

`10 / 2`

| Division

`5 * 2`

| Multiplication

`3 ** 2`

| Raise `3`

to the power of `2`

(or \(3^{2}\))

`27 ** (1/3)`

| The `3`

rd root of `27`

(or \(\sqrt[3]{27}\))

`x += 1`

| Assign the value of `x + 1`

to `x`

`x -= 1`

| Assign the value of `x - 1`

to `x`

## Lists

`l = [100, 21, 88, 3]`

| Assign a list containing the integers `100`

, `21`

, `88`

, and `3`

to the variable `l`

`l = list()`

| Create an empty list and assign the result to `l`

`l[0]`

| Return the first value in the list `l`

`l[-1]`

| Return the last value in the list `l`

`l[1:3]`

| Return a slice (list) containing the second and third values of `l`

`len(l)`

| Return the number of elements in `l`

`sum(l)`

| Return the sum of the values of `l`

`min(l)`

| Return the minimum value from `l`

`max(l)`

| Return the maximum value from `l`

`l.append(16)`

| Append the value `16`

to the end of `l`

`l.sort()`

| Sort the items in `l`

in ascending order

`" ".join(["A", "B", "C", "D"])`

| Converts the list `["A", "B", "C", "D"]`

into the string `"A B C D"`

## Dictionaries

`d = {"CA": "Canada", "GB": "Great Britain", "IN": "India"}`

| Create a dictionary with keys of `"CA"`

, `"GB"`

, and `"IN"`

and corresponding values of of `"Canada"`

, `"Great Britain"`

, and `"India"`

`d["GB"]`

| Return the value from the dictionary `d`

that has the key `"GB"`

`d.get("AU","Sorry")`

| Return the value from the dictionary `d`

that has the key `"AU"`

, or the string `"Sorry"`

if the key `"AU"`

is not found in `d`

`d.keys()`

| Return a list of the keys from `d`

`d.values()`

| Return a list of the values from `d`

`d.items()`

| Return a list of `(key, value)`

pairs from `d`

## Modules and Functions

*The body of a function is defined through indentation*

`import random`

| Import the module `random`

`from random import random`

| Import the function `random`

from the module `random`

```
def calculate(addition_one,addition_two,exponent=1,factor=1):
result = (value_one + value_two) ** exponent * factor
return result
```

Define a new function `calculate`

with two required and two optional named arguments which calculates and returns a result.

`addition(3,5,factor=10)`

| Run the addition function with the values `3`

and `5`

and the named argument `10`

## Boolean Comparisons

`x == 5`

| Test whether `x`

is equal to `5`

`x != 5`

| Test whether `x`

is not equal to `5`

`x > 5`

| Test whether `x`

is greater than `5`

`x < 5`

| Test whether `x`

is less than `5`

`x >= 5`

| Test whether `x`

is greater than or equal to `5`

`x <= 5`

| Test whether `x`

is less than or equal to `5`

`x == 5 or name == "alfred"`

| Test whether `x`

is equal to `5`

or `name`

is equal to `"alfred"`

`x == 5 and name == "alfred"`

| Test whether `x`

is equal to `5`

and `name`

is equal to `"alfred"`

`5 in l`

| Checks whether the value `5`

exists in the list `l`

`"GB" in d`

| Checks whether the value `"GB"`

exists in the keys for `d`

## Statements and Loops

*The body of if statements and loops are defined through indentation*

```
if x > 5:
print("{} is greater than five".format(x))
elif x < 0:
print("{} is negative".format(x))
else:
print("{} is between zero and five".format(x))
```

Test the value of the variable `x`

and run the code body based on the value

```
for value in l:
print(value)
```

Iterate over each value in `l`

, running the code in the body of the loop with each iteration.

```
while x < 10:
x += 1
```

Run the code in the body of the loop until the value of `x`

is no longer less than `10`

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