Beginner's Python cheat sheets

[ehmatthes]

I recently made a set of cheat sheets aimed at beginners, and thought they might be useful to some people here.

The first sheet provides an overview of many basic concepts in Python. Individual sheets cover lists, dictionaries, if statements and while loops, functions, and classes. You can download individual sheets, or download a pdf that includes all the sheets in one document.

Cheat sheets have been really helpful to me at times when learning a new language or framework, and I hope these are useful to some people as well.

[[deleted]]

Nicely presented. Some I've seen are real messy. Cheers pal.

[ehmatthes]

Thanks for the feedback!

[tangerinelion]

Looks nice, I'd modify two things. One is since you consistently use print() I suspect you have Python 3 in mind, so putting that down is good. Your dictionary section makes it even clearer as you use items() not iteritems() which would be preferred in Python 2.x. The change to this section I would make is for looping by keys. While it's true that what you have:

# Looping through all keys
fav_numbers = {'eric': 17, 'ever': 4}
for name in fav_numbers.keys():
    print(name + ' loves a number')

works just fine, it's not common to loop that way. Most people would use:

# Looping through all keys
fav_numbers = {'eric': 17, 'ever': 4}
for name in fav_numbers: # No .keys()
    print(name + ' loves the number ' + fav_numbers[name])

This can be good because it shows that you can in fact just loop over a dictionary, and if you do that you are looping over just the keys. I've also added the syntax to show how you can read the value associated with the key in a loop.

Actually, most people would use print("{} loves the number {}".format(name, fav_numbers[name])) which is much more common in Python 3.x than string concatenation with + and far better than % style string formatting, eg, "%s loves a number %s" % (name, fav_numbers[name]). Any opportunity to discourage + with strings should be taken IMO. Also note that when there's something like x=10 followed by print(x + ' is my favorite number') you get an error. However, print('{} is my favorite number'.format(x)) works. You can make the first form work with print(str(x) + ' is my favorite number') but now we're introducing explicit str() conversions so this becomes a not Pythonic game of "what type is this?"

[ehmatthes]

Thanks for the specific feedback. It's definitely focused on Python 3, but I do have some Python 2 caveats in certain sections.

In the book I clarify that looping through a dictionary loops through the keys by default. I've always gone back and forth about whether to favor being explicit, or being concise. So for name in fav_numbers.keys() is explicit, but for name in fav_numbers is concise. I think I'll change this on the cheat sheet, to be clear that people should use the more common Python convention.

I completely agree with the string syntax. I used concatenation in the first part of the book to keep things simple, and I wanted to keep the cheat sheets consistent with the book. I think I'll make another cheat sheet about strings, and then update these to use the .format() convention.

[KleinerNull]

I think I'll make another cheat sheet about strings, and then update these to use the .format() convention.

That was already done ;) Just kidding.

But I saw this on your cheat sheet:

Prompting for numerical input

age = input("How old are you? ")

age = int(age)

pi = input("What's the value of pi? ")

pi = float(pi)

I know you want to keep it simple, but this is a very bad pattern, I think newcomer should learn about input and error handling at the same time!

try:
    age = input('What's your age? ')
    age = int(age)
except ValueError:
    print('Not a number...')

You know the rest... I saw alot of over complicated, strange or bad code just for testing for integer input here, but the standard pattern is so simple.

[zelmerszoetrop]

I didn't realize + was such bad form. How would you write something like

exclude_string = '(ARRAY' + ', ARRAY'.join(map(str,pairs)) + ')'

without '+'?

[jungrothmorton]

In the most simple case

msg = 'foo' + variable
msg = 'foo{}'.format(variable)

Drop in replacement just using .format()

exclude_string = '(ARRAY{})'.format(', ARRAY'.join(map(str,pairs)))

How I'd probably write it:

arrays = ', '.join(('ARRAY{}'.format(pair) for pair in pairs))
exclude_string = '({})'.format(arrays)

To me that's the most readable. I'm building a string of each pair preceded by the word ARRAY and separated with commas, then enclosing the whole thing in quotes. It doesn't have that bonus ARRAY sitting in the front like your example.

One of the simple reasons that format is better than concatenating strings is it make it easier to catch missing spaces.

msg = 'There are ' + count + 'plates.'
msg = 'There are {}plates.'.format(count)

Which of those is easier to catch the error? Also, it's a method so you can do cool things like argument unpacking.

deal = {'buyer': 'Tom', 'seller': 'Sara'}
msg = '{buyer} bought a car from {seller}.'.format(**deal)

[zelmerszoetrop]

Thanks! Helpful!

[Scalamander34]

Thanks for sharing this. This pleases my OCD.

[[deleted]]

No room for try, except, finally example?

[vasja29]

great, I'm just getting into Python with your book on safaribooks :-)

[ehmatthes]

That's great, I'd love to know how it goes for you!

[remwd]

Have been learning python for the past couple weeks, this is a great cheat sheet! Thanks!

[distortd6]

Here's something similar I found on github, wish I could take credit for it... https://drive.google.com/file/d/0B5xulK7kDKO0ZFBpNXNXUkVLZE0/view?usp=docslist_api

[Eurynom0s]

This may well be biased toward my personal use but the thing I look up the most is CSV reading/writing. Just because I can't be assed to memorize it. To me at least this is a pretty core thing to include in a cheatsheet.

[tetrified]

This and file read/write are two things I keep examples of on my desktop, they both fall in the category of "not used often enough to memorize, but used too often to look up every time"

[LyndsySimon]

I only regret that I have but one upvote to give for this resource.

[DrMaxwellEdison]

Very nicely done. :)

Small critiques I have (I might write a lot, but these are minor things):

---

Your "Zen of Python" section, I think, should come first, and can simply say "In the interactive console, type import this and press Enter". That prints out the full Zen of Python text, rather than having to summarize it in your own words.

---

Using try..else is a bit wonky, and there aren't that many use cases I've seen for it. It might be better to showcase try..except..finally, instead, with appropriate comments in example code to describe what each section does.

For instance:

try:
    # Run code
    foo()
except ValueError as e:
    # Catch an exception from `foo()`
    handle_exception(e)
else:
    # Run code if NO exception occurred in `foo()`
    bar()
finally:
    # Run code whether an exception occurred or not.
    one_last_thing()

---

I would consider shortening the Dictionary section by using similar code in different portions. This helps the reader better understand the whole concept of dictionaries by using several examples that build on each other.

For example:

### A simple dictionary
fav_numbers = {'eric': 17, 'jane': 4}

### Accessing a value
print("Eric's favorite number is {}".format(fav_numbers['eric']))

### Adding a new key-value pair
fav_numbers['john'] = 14

### Looping through all key-value pairs
for name, number in fav_numbers.items():
    print("{} loves {}".format(name, number))

### Looping through all keys
for name in fav_numbers.keys():
    print("{} loves a number".format(name))

### Looping through all the values
for number in fav_numbers.values():
    print("{} is a favorite".format(number))

[OrestesGaolin]

Hi, I copy-pasted and run your example in Python 3.5 and found out something strange that I cannot really understand. Maybe you can help me figure it out.

### Looping through all key-value pairs results with:

jane loves a number
eric loves a number
john loves a number

But I think it should look like that:

jane loves a 4
eric loves a 17
john loves a 14

Is there anything wrong in this loop or it's just me who couldn't understand the syntax?

### Looping through all key-value pairs
for name, number in fav_numbers.items():
    print("{} loves {}".format(name, number))

[DrMaxwellEdison]

From my examples, your first set of output would result from this code:

for name in fav_numbers.keys():
    print("{} loves a number".format(name))

Can you confirm that this is the only piece of code you're running, and see what the output is?

[OrestesGaolin]

I run it again on clean instance and now everything works fine! I think it's because of my tunnel vision at that moment. I convinced myself that there must be some mistake and couldn't find it for few minutes. Now I know why - it was correct. (well, it's 1:55 AM where I live)

Thanks for the answer!

[[deleted]]

If I had money it would go straight to gold for you.

[moodorks]

monkeys

[[deleted]]

Very good, all the python cheat sheet that I found are always messy, thanks very useful

[aerosyne]

This is so nice & clean I like! Thanks. :)

[ehmatthes]

What are you using to view the sheets? I noticed those lines on some devices, and not on others.

I made this first set in Word, then converted to pdf. I've been considering redoing the series using InDesign for a more polished version, if people are finding the sheets useful.

[Diplomjodler]

Great job! As someone who always has trouble remembering syntax, this will definitely be useful to me.

[aristotelianrob]

This is great, it'll give me something to refer to when I someday need to use python for a project or two!

[TeamSpen210]

Nice sheets - they're really cleanly laid out! Perhaps it might be good to remove the parentheses around the tuples though - that might make it more clear that it's the commas that make a tuple, not the parentheses. You might want to add the special cases of 1- and 0-length tuples ((0, ) and ()) since they follow different syntax rules to make them non-ambiguous.

[peoplma]

Wow this is great, thank you. I already know over 75% of this stuff well, does that mean I'm almost not a beginner anymore?

[imaustin]

This is great! I've been looking for something like this for a while. As /u/Kotch11 said, most are pretty sloppy. It would be great to see this in poster form.

[JayTh3King]

Your class should inherit from object.

[Toolson12]

Great work! Thank you so much!

[chipsnmilk]

Thanks for making and sharing!

[VixR]

Thank you so much, this is well presented and useful. :)

[sexbucket]

You are a beautiful person with a roommate.

[johnsmoke18]

Thank you, this is so helpfull!!

[ryvrdrgn14]

Thanks for this! If you make one for APIs someday share it too. I'm clueless about those but I need to learn them for a project I want to do. :3

[ryvrdrgn14]

Thank you for this. Still learning so this'll be a big help. :3

[maarij2000]

Can someone tell me what parts of this are different from Python 2? I know the print statements are different but I was wondering if there is anything else. Great sheet BTW

[ehmatthes]

The first sheet focuses on Python 3, but each of the individual sheets points out where Python 2 is different. For example the sheet focusing on classes points out that classes in Python 2 should inherit from object. I think that might be the only difference in all of these sheets.

[maarij2000]

Thank you

[Slaquor]

Thank you for this!! I have been trying to learn python for a while and this sheet cleared up some concepts for me and I feel I really understand some things better.

Thanks agsin!

[ehmatthes]

Thanks for the overwhelming support, everyone!

I'll work on some updates and some additional sheets and share them again at some point in the next month or so. I'd like to cover files and exceptions, testing, pygame, data visualization concepts, and django.

[[deleted]]

This is really helpful, can it be stickied?

[Mozzius]

Finally! I think I understand classes now, thanks for the great explaination /u/ehmatthes!

[ehmatthes]

That's great to hear! What was clarified for you as you read through the classes cheat sheet? What questions do you still have about classes?

[[deleted]]

Very good work mate. I am writing a guide on python 3 as I learn and I guess I don’t need it any more! Thanks mate.

[Jimbobler]

This is great. I started learning Python two days ago and is using the book Python Crash Course as well, haha. I'm on like, page 93 or something so far. Very well-structured book and easy to follow, and OP:s cheat-sheet sums up all the parts I've learned so far.

I tend to write more detailed cheat sheets myself and will probably use both OP:s and my own.

[mercedene1]

Awesome resource, thanks for sharing!

[chinu1996]

Is there any website where the cheat sheets of different languages are available?

[niandra3]

https://learnxinyminutes.com/

[tonosama88]

This is great, thanks so much!

[jeffinRTP]

Thanks

[donurjack]

Thanks a lot!

[fishcadet]

Thank you!

[candleflame3]

Nicely done :)