pip 安装报错 "unable to get local issuer certificate" 解决

在 CentOS 7 上使用 pip 安装 Python 包时出现证书验证错误：

$ pip install tensorboard
Looking in indexes: https://pypi.mirrors.ustc.edu.cn/simple
WARNING: Retrying (Retry(total=4, connect=None, read=None, redirect=None, status=None)) after connection broken by 'SSLError(SSLCertVerificationError(1, '[SSL: CERTIFICATE_VERIFY_FAILED] certificate verify failed: unable to get local issuer certificate (_ssl.c:1007)'))': /simple/tensorboard/
WARNING: Retrying (Retry(total=3, connect=None, read=None, redirect=None, status=None)) after connection broken by 'SSLError(SSLCertVerificationError(1, '[SSL: CERTIFICATE_VERIFY_FAILED] certificate verify failed: unable to get local issuer certificate (_ssl.c:1007)'))': /simple/tensorboard/
WARNING: Retrying (Retry(total=2, connect=None, read=None, redirect=None, status=None)) after connection broken by 'SSLError(SSLCertVerificationError(1, '[SSL: CERTIFICATE_VERIFY_FAILED] certificate verify failed: unable to get local issuer certificate (_ssl.c:1007)'))': /simple/tensorboard/
WARNING: Retrying (Retry(total=1, connect=None, read=None, redirect=None, status=None)) after connection broken by 'SSLError(SSLCertVerificationError(1, '[SSL: CERTIFICATE_VERIFY_FAILED] certificate verify failed: unable to get local issuer certificate (_ssl.c:1007)'))': /simple/tensorboard/
WARNING: Retrying (Retry(total=0, connect=None, read=None, redirect=None, status=None)) after connection broken by 'SSLError(SSLCertVerificationError(1, '[SSL: CERTIFICATE_VERIFY_FAILED] certificate verify failed: unable to get local issuer certificate (_ssl.c:1007)'))': /simple/tensorboard/
Could not fetch URL https://pypi.mirrors.ustc.edu.cn/simple/tensorboard/: There was a problem confirming the ssl certificate: HTTPSConnectionPool(host='pypi.mirrors.ustc.edu.cn', port=443): Max retries exceeded with url: /simple/tensorboard/ (Caused by SSLError(SSLCertVerificationError(1, '[SSL: CERTIFICATE_VERIFY_FAILED] certificate verify failed: unable to get local issuer certificate (_ssl.c:1007)'))) - skipping
ERROR: Could not find a version that satisfies the requirement tensorboard (from versions: none)
Could not fetch URL https://pypi.mirrors.ustc.edu.cn/simple/pip/: There was a problem confirming the ssl certificate: HTTPSConnectionPool(host='pypi.mirrors.ustc.edu.cn', port=443): Max retries exceeded with url: /simple/pip/ (Caused by SSLError(SSLCertVerificationError(1, '[SSL: CERTIFICATE_VERIFY_FAILED] certificate verify failed: unable to get local issuer certificate (_ssl.c:1007)'))) - skipping
ERROR: No matching distribution found for tensorboard

问题原因：无法找到本地颁发者证书，个人认为是由于 CentOS 7 自带的证书过于老旧所致。

解决方法：手动安装证书并设置证书路径

pip install certifi
export SSL_CERT_FILE=$(python -m certifi)

参考：SSL: CERTIFICATE_VERIFY_FAILED certificate verify failed: unable to get local issuer certificate (_ssl.c:1129)'))) | Stack Overflow