检查是否安装了Python软件包

What’s a good way to check if a package is installed while within a Python script? I know it’s easy from the interpreter, but I need to do it within a script.

I guess I could check if there’s a directory on the system that’s created during the installation, but I feel like there’s a better way. I’m trying to make sure the Skype4Py package is installed, and if not I’ll install it.

My ideas for accomplishing the check

• check for a directory in the typical install path
• try to import the package and if an exception is throw, then install package

If you mean a python script, just do something like this:

Python 3.3+ use sys.modules and find_spec:

import importlib.util
import sys

# For illustrative purposes.
name = 'itertools'

if name in sys.modules:
    print(f"{name!r} already in sys.modules")
elif (spec := importlib.util.find_spec(name)) is not None:
    # If you choose to perform the actual import ...
    module = importlib.util.module_from_spec(spec)
    sys.modules[name] = module
    spec.loader.exec_module(module)
    print(f"{name!r} has been imported")
else:
    print(f"can't find the {name!r} module")

Python 3:

try:
    import mymodule
except ImportError as e:
    pass  # module doesn't exist, deal with it.

Python 2:

try:
    import mymodule
except ImportError, e:
    pass  # module doesn't exist, deal with it.

Updated answer

A better way of doing this is:

import subprocess
import sys

reqs = subprocess.check_output([sys.executable, '-m', 'pip', 'freeze'])
installed_packages = [r.decode().split('==')[0] for r in reqs.split()]

The result:

print(installed_packages)

[
    "Django",
    "six",
    "requests",
]

Check if requests is installed:

if 'requests' in installed_packages:
    # Do something

Why this way? Sometimes you have app name collisions. Importing from the app namespace doesn’t give you the full picture of what’s installed on the system.

Note, that proposed solution works:

• When using pip to install from PyPI or from any other alternative source (like pip install http://some.site/package-name.zip or any other archive type).
• When installing manually using python setup.py install.
• When installing from system repositories, like sudo apt install python-requests.

Cases when it might not work:

• When installing in development mode, like python setup.py develop.
• When installing in development mode, like pip install -e /path/to/package/source/.

Old answer

A better way of doing this is:

import pip
installed_packages = pip.get_installed_distributions()

For pip>=10.x use:

from pip._internal.utils.misc import get_installed_distributions

Why this way? Sometimes you have app name collisions. Importing from the app namespace doesn’t give you the full picture of what’s installed on the system.

As a result, you get a list of pkg_resources.Distribution objects. See the following as an example:

print installed_packages
[
    "Django 1.6.4 (/path-to-your-env/lib/python2.7/site-packages)",
    "six 1.6.1 (/path-to-your-env/lib/python2.7/site-packages)",
    "requests 2.5.0 (/path-to-your-env/lib/python2.7/site-packages)",
]

Make a list of it:

flat_installed_packages = [package.project_name for package in installed_packages]

[
    "Django",
    "six",
    "requests",
]

Check if requests is installed:

if 'requests' in flat_installed_packages:
    # Do something

As of Python 3.3, you can use the find_spec() method

import importlib.util

# For illustrative purposes.
package_name = 'pandas'

spec = importlib.util.find_spec(package_name)
if spec is None:
    print(package_name +" is not installed")

If you want to have the check from the terminal, you can run

pip3 show package_name

and if nothing is returned, the package is not installed.

If perhaps you want to automate this check, so that for example you can install it if missing, you can have the following in your bash script:

pip3 show package_name 1>/dev/null #pip for Python 2
if [ $? == 0 ]; then
   echo "Installed" #Replace with your actions
else
   echo "Not Installed" #Replace with your actions, 'pip3 install --upgrade package_name' ?
fi

As an extension of this answer:

For Python 2.*, pip show <package_name> will perform the same task.

For example pip show numpy will return the following or alike:

Name: numpy
Version: 1.11.1
Summary: NumPy: array processing for numbers, strings, records, and objects.
Home-page: http://www.numpy.org
Author: NumPy Developers
Author-email: numpy-discussion@scipy.org
License: BSD
Location: /home/***/anaconda2/lib/python2.7/site-packages
Requires: 
Required-by: smop, pandas, tables, spectrum, seaborn, patsy, odo, numpy-stl, numba, nfft, netCDF4, MDAnalysis, matplotlib, h5py, GridDataFormats, dynd, datashape, Bottleneck, blaze, astropy

You can use the pkg_resources module from setuptools. For example:

import pkg_resources

package_name = 'cool_package'
try:
    cool_package_dist_info = pkg_resources.get_distribution(package_name)
except pkg_resources.DistributionNotFound:
    print('{} not installed'.format(package_name))
else:
    print(cool_package_dist_info)

Note that there is a difference between python module and a python package. A package can contain multiple modules and module’s names might not match the package name.

Open your command prompt type

pip3 list

I’d like to add some thoughts/findings of mine to this topic. I’m writing a script that checks all requirements for a custom made program. There are many checks with python modules too.

There’s a little issue with the

try:
   import ..
except:
   ..

solution. In my case one of the python modules called python-nmap, but you import it with import nmap and as you see the names mismatch. Therefore the test with the above solution returns a False result, and it also imports the module on hit, but maybe no need to use a lot of memory for a simple test/check.

I also found that

import pip
installed_packages = pip.get_installed_distributions()

only the packages has been installed with pip. On my system pip freeze returns over 40 python modules, while installed_packages has only 1, the one I installed manually (python-nmap).

Another solution below that I know it may not relevant to the question, but I think it’s a good practice to keep the test function separate from the one that performs the install it might be useful for some.

The solution that worked for me. It based on this answer How to check if a python module exists without importing it

from imp import find_module

def checkPythonmod(mod):
    try:
        op = find_module(mod)
        return True
    except ImportError:
        return False

NOTE: this solution can’t find the module by the name python-nmap too, I have to use nmap instead (easy to live with) but in this case the module won’t be loaded to the memory whatsoever.

If you’d like your script to install missing packages and continue, you could do something like this (on example of ‘krbV’ module in ‘python-krbV’ package):

import pip
import sys

for m, pkg in [('krbV', 'python-krbV')]:
    try:
        setattr(sys.modules[__name__], m, __import__(m))
    except ImportError:
        pip.main(['install', pkg])
        setattr(sys.modules[__name__], m, __import__(m))

A quick way is to use python command line tool. Simply type import <your module name> You see an error if module is missing.

$ python
Python 2.7.6 (default, Jun 22 2015, 17:58:13) 
>>> import sys
>>> import jocker
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
ImportError: No module named jocker
$

Hmmm … the closest I saw to a convenient answer was using the command line to try the import. But I prefer to even avoid that.

How about ‘pip freeze | grep pkgname’? I tried it and it works well. It also shows you the version it has and whether it is installed under version control (install) or editable (develop).

You can use this:

class myError(exception):
 pass # Or do some thing like this.
try:
 import mymodule
except ImportError as e:
 raise myError("error was occurred")

In the Terminal type

pip show some_package_name

Example

pip show matplotlib

I would like to comment to @ice.nicer reply but I cannot, so … My observations is that packages with dashes are saved with underscores, not only with dots as pointed out by @dwich comment

For example, you do pip3 install sphinx-rtd-theme, but:

• importlib.util.find_spec(sphinx_rtd_theme) returns an Object
• importlib.util.find_spec(sphinx-rtd-theme) returns None
• importlib.util.find_spec(sphinx.rtd.theme) raises ModuleNotFoundError

Moreover, some names are totally changed. For example, you do pip3 install pyyaml but it is saved simply as yaml

I am using python3.8

if pip3 list | grep -sE '^some_command\s+[0-9]' >/dev/null
  # installed ...
else
  # not installed ...
fi

Go option #2. If ImportError is thrown, then the package is not installed (or not in sys.path).