Title

class: center, middle

# Django and Python 3

Pete Graham

[petegraham.co.uk/slides/django-and-python-3/](http://petegraham.co.uk/slides/django-and-python-3/)

[@petexgraham](https://twitter.com/petexgraham)

[pete@petegraham.co.uk](mailto:pete@petegraham.co.uk)

---

# Coming up

* Introduction
* Why upgrade to Python 3
* Thing to do first
* Steps
* Installing Python 3
* Case study
* Questions

---

# Introduction

* [http://petegraham.co.uk/the-python-3-problem-and-swift/](http://petegraham.co.uk/the-python-3-problem-and-swift/)
* We all use Django
* We don't all use Python 3 (in production)
* How difficult is it to port an existing site?

---

#Why upgrade to Python 3?

1. It's a better language
2. Technical debt

---

#Thing to do first

* Upgrade to newest Django or LTS
* Fix security vunerabilities
* Decent deployment
* Decent tests

---

#Steps

1. Upgrade Django
2. Upgrade other libraries to latest Python 2.7 versions
3. Run 2 to 3 migrations tool
4. Run site on Python 3

---

#Installing Python 3

[https://www.python.org/downloads/](https://www.python.org/downloads/)

```bash
mkvirtualenv --python=/usr/local/bin/python3 venv_name
```

---

# Case study

* Smallish site
* Originally made in 2013
* Django 1.5

---

# Functionality

* Multi-step application process
* Email sending
* Django Admin for CMS
* Export Applications as PDF

---

# requirements.txt

```bash
Django==1.8.3
Fabric==1.10.2
Pillow==2.9.0
django-admin-sortable==1.8.4
ipython==4.0.0
paramiko==1.15.2
psycopg2==2.6.1
pycrypto==2.6.1
python-Levenshtein==0.12.0
wsgiref==0.1.2
pywurfl==7.2.1
singledispatch==3.4.0.3
django-user-agents==0.3.0
xhtml2pdf==0.0.6
```

---

# Can I Use Python 3?

```bash
pip install caniusepython3
caniusepython3 -r requirements.txt

...

You need 4 projects to transition to Python 3.
Of those 4 projects, 4 have no direct dependencies blocking their transition:

django-admin-sortable
  fabric
  pywurfl
  xhtml2pdf
```

---

# Package 1/4 fabric

* Runs on laptop
* New version out soon
* [https://gist.github.com/mok0/99b51e95ceeb4f3fd28b](https://gist.github.com/mok0/99b51e95ceeb4f3fd28b)

---

# Package 2/4 django-admin-sortable

* 1.7.1 and higher are compatible with Python 3
* caniusepython3 lied!

---

# Package 3/4 pywurfl

* [pywurfl][1] is a [Python][3] language package - [https://pypi.python.org/pypi/pywurfl/](https://pypi.python.org/pypi/pywurfl/)
* caniusepython3 lied again!

---

# Package 4/4 xhtml2pdf

* Some dicsussion on Github
* It might work...

---

# Python installing requirements

```bash
pip install -r requirements.txt

...
blah blah pip is installing packages
...

Collecting wsgiref==0.1.2 (from -r requirements.txt (line 10))
  Downloading wsgiref-0.1.2.zip
    Complete output from command python setup.py egg_info:
    Traceback (most recent call last):
      File "<string>", line 20, in <module>
      File "/private/var/folders/ny/0wbl966d11v2j9brxg1vtqy00000gn/T/pip-build-vaxm8il7/wsgiref/setup.py", line 5, in <module>
        import ez_setup
      File "/private/var/folders/ny/0wbl966d11v2j9brxg1vtqy00000gn/T/pip-build-vaxm8il7/wsgiref/ez_setup/__init__.py", line 170
        print "Setuptools version",version,"or greater has been installed."
                                 ^
    SyntaxError: Missing parentheses in call to 'print'

----------------------------------------
Command "python setup.py egg_info" failed with error code 1 in /private/var/folders/ny/0wbl966d11v2j9brxg1vtqy00000gn/T/pip-build-vaxm8il7/wsgiref
```

---

#Remove redundant requirements

---

# Getting the site running

```bash
django-admin runserver

...

File "/Users/pxg/Projects/graduates/graduates/applicant/admin.py", line 6, in <module>
    from graduates.applicant.download_as_csv import download_as_csv
  File "/Users/pxg/Projects/graduates/graduates/applicant/download_as_csv.py", line 103, in <module>
    @download_as_csv.register(basestring)
NameError: name 'basestring' is not defined
```

---

# Remove obsolete code

---

# Getting the site running...

* Run the dev server again
* Getting a syntax error from xhtml2pdf

---

#Latest versions not always on Pypi

```bash
pip uninstall xhtml2pdf
```

Add to requirements.txt
```bash
-e git://github.com/chrisglass/xhtml2pdf.git#egg=xhtml2pdf
```

---

#Encoding Error

Happened after submitting the first step of the form.

```python
TypeError at /apply
Unicode-objects must be encoded before hashing
```

Culprit code
```python
instance.hash = hashlib.sha1(string).hexdigest()
```
Fix
```python
instance.hash = hashlib.sha1(string.encode('utf-8')).hexdigest()
```
---

#basestring Error

Happened after submitting step 3 on form. Step 4 is degree information.

```python
name 'basestring' is not defined
```

Culprit code
```python
if isinstance(value, basestring):
```

Fix
```python
from six import string_types
if isinstance(value, string_types):
```

[https://djangosnippets.org/snippets/1688/](https://djangosnippets.org/snippets/1688/)

---

# dict_items Error

Also happend after submitting step 3 of the form. Same snippet.

```python
'dict_items' object has no attribute 'append'
```

Culprit code
```python
month_choices = MONTHS.items()
```

Fix
```python
month_choices = list(MONTHS.items())
```

---

# "Object" Errors

Also on step 4. University drop-down.

Culprit code
```python
def __unicode__(self):
    return self.name
```

Fix
```python
def __str__(self):
    return self.name
```
---

# str buffer interface Errors

Happened on download applicants PDF.

```python
'str' does not support the buffer interface
```

Culprit code
```python
from cStringIO import StringIO
```

Fix
```python
from io import StringIO
```

---

# iter.next() error

Also happened on download applicants PDF.

Culprit code
```python
counter.next()
```

Fix
```python
next(counter)
```

---

# string argument expected, got 'bytes'

Happened generating Zip file in memory error for applicants PDF.

Change `StringIO` to `ByteIO`. Or can be done as an import.

```python
from io import ByteIO as StringIO
```

---

#Conclusion

* Start new projects with Python 3
* Not all packages support Python 3
* Port Python 2 projects if you can justify it
* Most complex code to debug was copy and pasted

---

# Questions?

---

# Thank you

[https://petegraham.co.uk/slides/django-and-python-3/](https://petegraham.co.uk/slides/django-and-python-3/)

[@petexgraham](https://twitter.com/petexgraham)

[pete@petegraham.co.uk](mailto:pete@petegraham.co.uk)

---